Ruby & Sapphire cover

Burma ruby, Mogok, ruby, sapphire, corundum, gems, gemology

Note: The following is only one of forty-five studies of world sources found in Chapter 12 of Richard Hughes' book, Ruby & Sapphire. If you like what you see, order a copy direct from the publisher.

Burma Continued from Part 2

Burma ruby, Mogok, ruby, sapphire, corundum, gems, gemology

Figure 36. A fine 7.01-ct. Mogok ruby. (Stone: Jan Goodman; photo: Tino Hammid)

 

Pigeon's blood: Chasing the elusive Burmese bird

The Burmese term for ruby is padamya (`plenty of mercury'). Other terms for ruby are derived from the word for the seeds of the pomegranate fruit.13 Traditionally, the Burmese have referred to the finest hue of ruby as "pigeon's blood" (ko-twe ), a term which may be of Chinese (Anonymous, 1943) or Arab origin. Witness the following from al-Akfani, who described thus the top variety of ruby:

Rummani has the colour of the fresh seed of pomegranate or of a drop of blood (drawn from an artery) on a highly polished silver plate.

al-Akfani, ca. 1348 AD (from Sarma, 1984)

Some have compared this color to the center of a live pigeon's eye (Brown & Day, 1955). Halford-Watkins described it as a rich crimson without trace of blue overtones (Anonymous, 1943). Others have defined this still further as the color of the first two drops of blood from the nose of a freshly slain Burmese pigeon. But the piece de resistance of pigeon's-blood research has to be that of James Nelson (1985; Nelson discourages use of such fanciful terms):

In an attempt to seek a more quantitative description for this mysterious red colour known only to hunters and the few fortunate owners of the best Burmese rubies, the author sought the help of the London Zoo. Their Research Department were quick to oblige and sent a specimen of fresh, lysed, aerated, pigeon's blood. A sample was promptly spectrophotometered…. The Burmese bird can at last be safely removed from the realms of gemmology and consigned back to ornithology.

James B. Nelson, 1985, Journal of Gemmology

After that, the only question remaining is whether or not "spectrophotometered" is a genuine English verb.

Color preferences do change with time. The preferred color today is not necessarily that of a hundred, or even fifty, years ago. In the author's experience, the color most coveted today is that akin to a red traffic signal or stoplight. It is a glowing red color, due to the strong red fluorescence of Burmese rubies, and is unequalled in the world of gemstones. Thai rubies may possess a purer red body color, but the lack of red fluorescence leaves them dull by comparison. It must be stressed that the true pigeon's-blood red is extremely rare, more a color of the mind than the material world. One Burmese trader expressed it best when he said "…asking to see the pigeon's blood is like asking to see the face of God." (Nordland, 1982)

The second-best color in Burma is termed "rabbit's blood," or yeong-twe. It is a slightly darker, more bluish red. Third best is a deep hot pink termed bho-kyaik. This was the favorite color of the famous Mogok gem dealer, A.C.D. Pain. U Thu Daw, longtime Mogok dealer and a contemporary of Pain's, has stated that bho-kyaik is not so much a color term, as an overall quality description. To qualify, a ruby must fulfill six requirements. First, it must be at least one carat. Second, the color must be of the third quality (exceeded only by ko-twe and yeong-twe ). The table facet must be perpendicular to the c axis, it must be well cut, of good luster and eye clean. The literal meaning of bho-kyaik is "preference of the British" (U Hla Win, pers. comm., 2 May, 22 June, 1994).

Fourth-best is a light pink color termed leh-kow-seet (literally `bracelet-quality' ruby). At the bottom of the ruby scale is the dark red color termed ka-la-ngoh. This has an interesting derivation for it means literally either "crying-Indian quality" or "even an Indian would cry," so termed because it was even darker than an Indian's skin. Most dark rubies were sold in Bombay or Madras, India. Ka-la-ngoh stones were said to be so dark that even Indians would cry out in despair when confronted with this quality.

Burma ruby, Mogok, ruby, sapphire, corundum, gems, gemology

Figure 37. U Hmat, the "Ruby King," at the town of Mogok, in Burma. (From O'Connor, 1905) According to O'Connor…

U Hmat was great here in the days before any Englishman had come within sight of Mogôk. He is not a foreigner… but a native of the soil. He lives some distance from the market-place in a rambling wooden house on piles…. At one end he has built himself a strong-room of brick, in which lie hidden, according to popular tradition, rubies of extraordinary value. U Hmat is seldom seen abroad. He goes, it is said, in terror of his life; and his courtyard is thronged with retainers, who make for him a kind of personal bodyguard. But in bygone days he travelled every year to Mandalay with a present of rubies, and was received in audience by the king. He is a builder of many monasteries and pagodas; but is said to be less lavish in this respect than most of his compatriots in Burma. He is believed accordingly by his European neighbours to have `his head screwed on the right way.' His character for economy is the topic of very favourable discussion at the dinnertables of the settlement, and it is a commonplace of opinion that he is the only Burman at the mines who is not a fool. Let it be added that he is the father of a pretty daughter, whose jewels are the despair of every other woman in Mogôk, and that he keeps her in strict seclusion, lest some adventurous youth should steal away her heart, or her person, or both. He has been good enough, however, to show me some of her most beautiful jewels.

V.C. Scott O'Connor, 1905, The Silken East

Burmese rubies compared

Until the discoveries in Vietnam in the late 1980s, Burmese rubies were without peer. Other sources, such as Kenya and Afghanistan, produced the occasional stone which could stand with Burma's best, but such stones were extremely rare. Discovery of ruby in Vietnam changed all that. For the first time in hundreds of years, a viable alternative to Burma presented itself. Only time will tell if the Vietnamese mines can continue to produce, but, historically speaking, Burmese rubies are in a class by themselves.

The color of a fine Burmese ruby is due to a combination of two factors. First, the best stones have high color intensity. This results from a mixture of the slightly bluish red body color and the purer red fluorescent emission. It is this red fluorescence which is the key, for it tends to cover up the dark areas of the stone caused by extinction from cutting. Thai rubies possess a purer red body color,14 but lack the strong fluorescence. In Thai rubies, where light is properly reflected off pavilion facets (internal brilliance), the color is good. However, where facets are cut too steep, light exits through the side instead of returning to the eye, creating darker areas (extinction). All stones possess this extinction to a certain degree, but in fine Burmese rubies, the strong crimson fluorescence masks it. The best Burmese stones actually glow red and appear as though Mother Nature brushed a broad swath of fluorescent red paint across the face of the stone. This is the carbuncle of the ancients, a term derived from the glowing embers of a fire.

Burma ruby, Mogok, ruby, sapphire, corundum, gems, gemology

Figure 38. A large crystal of calcite in an unheated Mogok ruby, in polarized light. Calcite is suggested because of the intersecting twinning planes visible within the included crystal. Such calcite crystals containing repeated glide twinning are often seen in Burmese rubies, which were formed in a calcite (marble) matrix. (Photo by the author)

A second factor is the presence of silk. Tiny exsolved inclusions tend to scatter light onto facets that would otherwise be extinct. This gives the color a softness, as well as spreading it across a greater part of the gem's face. Thai/Cambodian rubies contain no rutile silk, and thus possess more extinction.

In actuality, rubies from most sources possess a strong red fluorescence and silk similar to those from Burma, with the Thai rubies being the exception. However, those from Sri Lanka are generally too pale in color, while, with other sources, such as Kenya, Pakistan and Afghanistan, material clean enough for faceting is rare. Thus the combination of fine color (body color plus fluorescence) and facetable material (i.e., internally clean) has put the Burmese ruby squarely atop the crimson mountain. Some old-timers consider Burma to be not just the best source, but the only source of stones fit to be called ruby. When one considers that today probably 90% or more of newly-mined rubies owe a good measure of their clarity and color to heat treatment, this statement does not seem so outlandish (unfortunately, most Burmese rubies are today heat treated).

Features of Mogok ruby

Mogok's famous rubies display a distinctive internal picture, often allowing separation from rubies of other sources. Typical are both euhedral (`well-formed') and rounded crystal grains, along with dense clouds of rutile silk. Rhombohedral twinning is common, as is straight/angular color zoning, at times in a swirled pattern termed treacle. Generally absent, or in small numbers only, are the fluid-filled inclusions so common in Thai/Cambodian and Sri Lankan rubies.15

Varieties and occurrence

Mogok rubies range from lightest pink, through bright red, to deep garnet-red. Most tend to be slightly purplish-red in hue position, and grade into purple and violet sapphires. Fine star rubies are also found. Twelve-rayed star rubies have been reported, but are extremely rare.

Mogok rubies are derived from a crystalline limestone (marble) matrix, resulting from either contact or regional metamorphism.

Solids

Crystalline solids of many types are characteristic of Mogok rubies. They typically form clusters of rounded and/or euhedral grains of a light color (or colorless), often concentrating in the center of the crystal. The most common guests are calcite, spinel, corundum, apatite, rutile and zircon.

Calcite is present as both rounded and angular rhombs, recognizable by its cleavage and polysynthetic glide-twin lamellae. Twinning striations may also be found in included corundum crystals, which occur as tabular or rounded individuals of extremely low relief. These corundums included in corundum typically show a terraced, or step-like, appearance from multiple development of the basal pinacoid. Spinel crystals occur as both octahedra or, more often, as rounded irregular forms of low relief.

In addition to lightly colored or colorless mineral inclusions are guests with distinctive colors. Primary rutile crystals of deep red color and metallic luster stand out in high relief. Their square outline and knee-shaped twin or prismatic habit indicate their identity. Bright to pale yellow, partly resorbed crystals of low relief may be apatite; Eduard Gübelin (pers. comm., May 5, 1994) has reported that apatites in Mogok rubies tend to be rounded, while apatites in Sri Lankan stones often show distinct faces.

Yellow crystals of high relief suggest sphalerite or sphene. Rounded, partly resorbed grains of olivine are pale green. Deep-green prisms of a vanadium-bearing amphibole, pargasite [NaCa2 Fe4 (Al,Fe) Al2 Si6 O22 (OH)2 ] have been seen by the author in one spectacular vanadium-colored Mogok specimen (courtesy of Valaya Rangsit, ca. 1985). Dark brown to opaque slabs/plates suggest phlogopite mica. Zircon is also found, with and without stress halos.

Primary cavities

Primary fluid-filled cavities are not particularly common in Mogok rubies. This is said to result from the metamorphic processes in which they grew, which combine extremely slow growth rates with a fluid-poor environment (Roedder, 1982).

Negative crystals in Mogok rubies exhibit similar faces and habits as their host. Typical examples show a terraced appearance made up of numerous steps, the result of alternating development of pinacoid and pyramid (or rhombohedron) faces. Some are well-formed, with flat faces, while others are rounded. Negative crystals can be separated from solids because negative crystals show the same orientation as their host. In other words, the pinacoid face of each negative crystal is exactly parallel to the same face of the host and to any other negative crystals present in the stone.

When seen, negative crystals in Mogok rubies are often two phase. Eppler (1976) identified the filling as gases containing hydrogen sulfide. This constituent was recognized by its odor when the gems were crushed, opening the cavities. He speculated that gas bubbles within the growth solution perched on a face as the crystal grew. This provided an obstacle to the growth at that point on the face, while adjacent areas continued to grow. Eventually the surrounding gem engulfed the bubble completely, trapping it while simultaneously creating the negative crystal.

Secondary cavities

Untreated Mogok rubies contain far fewer secondary fluid inclusions (healing fissures or fingerprints ) than rubies from Thailand/Cambodia, Sri Lanka or Kenya. Heat-treated Burmese rubies may, however, contain many secondary fluid inclusions formed during the heat treatment process.

Burma ruby, Mogok, ruby, sapphire, corundum, gems, gemology

Figure 39. While strong color zoning is rare in Mogok sapphires, it is common in the rubies from this area. A fine example is shown above, viewed parallel to the c axis. (Photo: Tony Laughter)

When secondary fluid inclusions are found in untreated Mogok rubies, they tend to be well healed, with angular negative-crystal pockets sometimes containing gas bubbles. Others may be fractures where little healing has occurred. Generally lacking are the intermediate-stage, lacy fingerprints with narrow fluid tubes common to rubies from Thailand/Cambodia, Sri Lanka and Kenya. Heat-treated Mogok rubies, however, contain far more fingerprints and secondary-fluid inclusions, making the identification of origin more difficult.

Growth zoning

Straight, angular growth zoning is common in Mogok rubies, as with rubies from sources other than Thailand/Cambodia. The zoning is always found parallel to crystal faces. When looking parallel to crystal faces, the bands of color line up into sharp narrow zones; however, in other directions they may appear in irregular swirls termed treacle, from their resemblance to the swirls in syrup.

Twin development

Rhombohedral twinning is common, and may feature long, white exsolved boehmite needles at intersecting twin junctions.

Exsolved solids

One of the most diagnostic features of Mogok rubies is the dense white clouds of exsolved rutile. At high temperatures, when atomic spacing is greater, titanium enters into solid solution with the host corundum. As the corundum cools, however, its crystal lattice contracts, literally squeezing the titanium atoms out of solution, where they join with oxygen atoms to form minute crystals of rutile (TiO2). This process is known among mineralogists as exsolution-- the unmixing of a solid solution. Because of constraints on their movement by the solid corundum host, titanium atoms are unable to travel large distances. Therefore, rather than forming large crystals, they migrate together to form thousands of tiny slender needles where space permits. For rutile in corundum, this space is parallel to the faces of the second-order hexagonal prism, intersecting in three directions at 60/120° in the basal plane.

At times, only long slender threads are visible, while in other cases knife or dart shapes appear. Closer examination reveals many of these to be twin crystals with tiny v-shaped re-entrant angles visible at the broad end. They are flattened so thin in the basal plane that when illuminated with a fiber optic light guide from above, bursts of iridescent colors are seen, due to the interference of light from these microscopically-thin mineral lances.

Rubies from Mogok usually contain at least some rutile silk. It is found in dense white clouds made up of relatively short individuals, whereas in Sri Lankan corundums the rutile silk tends to be longer and less densely woven.16

Along with the rutile silk in Mogok rubies are clouds of minute particles of an unknown nature. These particle clouds, like the silk, also appear to result from exsolution, and are arranged in an identical pattern. At times, it has been noticed that heat treatment removes the rutile silk, but not the particles. Thus, in some cases at least, they may be composed of a mineral other than rutile. Due to their arrangement, they also influence the star effect of asteriated gems. In asteriated gems where silk clouds consist mainly of particles, the star is diffuse and lacking in definition. Conversely, where clouds contain a preponderance of needles, the star possesses better definition. Both needle- and particle-dominated stars can be found in Burmese corundums.

Burma ruby, Mogok, ruby, sapphire, corundum, gems, gemology

Figure 40. Rounded crystal grains are a common feature of Mogok rubies, such as those seen above, which are probably apatite. (Photo: Wimon Manorotkul)

Boehmite

Mogok rubies display one additional type of exsolved needle inclusion: boehmite. Boehmite needles are long white inclusions which form at the junction of intersecting twinning planes and, as a result, lie parallel to faces of the rhombohedron {1011}. Where planes meet, they intersect at angles of 86.1° and 93.9° (three directions total, two in the same plane). If one understands the vast differences in orientation and appearance, there is little likelihood that boehmite needles be confused with rutile silk.

Boehmite results from pressure-induced exsolution. This pressure also is responsible for the gliding (slipping) of atomic planes, creating polysynthetic twins. Since pressure also causes stress fractures, low-grade corundum is generally filled with these twin planes and the accompanying boehmite needles.

Boehmite needles are often long, running completely across the stone. When intersecting in the above manner, they appear like a sort of lattice framework, or creation from Mother Nature's erector set. At times, close examination shows the appearance of narrow fluid fingerprints and frequently one observes narrow stress fractures extending outwards from the needles at 45° angles in a spiral fringing appearance. When twin planes run through secondary fluid inclusions, the boehmite needles often divide them into parallel sections.

Together, the rhombohedral twinning/boehmite needles combination provide one of the best methods of separation from the synthetic stone, for they are seen in a large percentage of natural corundums from all sources. Although rhombohedral twinning and boehmite needles have on rare occasions been found by the author in Verneuil synthetic corundum, curved growth lines and gas bubbles allow separation. Nothing resembling this combination occurs in flux synthetics, making it important in the battle against sophisticated factory products. Twinning is sometimes found in flux synthetics, but without the accompanying boehmite needles.

Within Mogok rubies, rhombohedral twinning with boehmite needles is seen, although not as often as in Thai/Cambodian rubies.

Properties of Mogok (Burma) ruby

Property

Description

Color range/phenomena

  • Colorless to a deep red; the red of Burmese rubies is generally more purple than Thai/Cambodian rubies; some stones are of a 'garnet' red color. Most are strongly fluorescent.
  • Six-rayed stars are common; 12-rays are known, but rare.
  • Color-change stones (colored by vanadium) are rarely found. These have a color change similar to the Verneuil synthetic.

Geologic formation

Found in metamorphosed crystalline limestones (marble) and secondary deposits derived from the same.

Crystal habit

Typically stubby crystals consisting of prism/pyramids terminated by pinacoid faces and modified by the rhombohedron. Crystals often display a terraced appearance due to oscillation between the pinacoid and rhombohedron. Triangular depressions may be seen on pinacoid faces.

RI & birefringence

n omega = 1.760–1.766; n epsilon = 1.768–1.774; Bire. = 0.008 to 0.009

Specific Gravity

~4.00

Spectra

Visible: Strong Cr spectrum; V spectrum has been seen on rare occasions.

Fluorescence

Strong to very strong red to orangy red (LW stronger than SW). Heat-treated gems sometimes show chalky fluorescence from colorless patches.

Other features

None reported

Inclusion types

Description

Solids

 

Various, often in dense concentrations, including:

  • Apatite, hexagonal prisms (Gübelin, 1973)
  • Calcite, transparent, often with rhombohedral glide twinning (Gübelin, 1969b)
  • Dolomite (Gübelin & Koivula, 1986)
  • Corundum (Gübelin, 1953)
  • Garnet (Gübelin, 1953)
  • Graphite flakes, black (Kammerling & Scarratt et al., 1994)
  • Mica (muscovite) (Gübelin, 1953)
  • Olivine (Gübelin, 1973)
  • Pargasite, bright green crystals (Gübelin, 1973)
  • Pyrite (Gübelin & Koivula, 1986)
  • Pyrrhotite (Gübelin & Koivula, 1986)
  • Rutile prisms (not silk), dark red to black (Gübelin, 1953)
  • Scapolite, well-shaped crystals (Kammerling & Scarratt et al., 1994)
  • Sphalerite, brown (Gübelin, 1973)
  • Sphene, yellow-orange, high dispersion (Gübelin, 1969b)
  • Spinel group minerals (Gübelin, 1953)
  • Sulfur (Fritsch & Rossman, 1990)
  • Zircon (Gübelin, 1953)

Cavities

(liquids/gases/solids)

  • Primary negative crystals (rare)
  • Secondary negative crystals (healed fractures) are rare, except in heated stones. They often lack the lovely `lacy' appearance of Sri Lankan stones; typically they have fluid-filled channels which are widely spaced.

Growth zoning

  • Straight, angular growth zoning parallel to the faces along which it formed; irregular `treacle' like swirls in other directions

Twin development

  • Growth twins of unknown orientation
  • Polysynthetic glide twinning on the rhombohedron

Exsolved solids

  • Rutile silk in dense clouds of (often, but not always) short needles, parallel to the hexagonal prism (3 directions at 60/120° ) in the basal plane
  • Boehmite, long white needles along intersecting rhombohedral twin planes (3 directions, 2 in one plane, at 86.1 and 93.9° )
a. This table is based on the author's own extensive experience, along with published reports of Eppler (1976), Fritsch & Rossman (1990), Gübelin (1973), Gübelin & Koivula (1986) and Kammerling & Scarratt et al. (1994).

Burmese sapphires

Although rubies are found with much greater frequency at Mogok (rubies form about 80–90% of the total output), sapphires may reach larger sizes. Cut gems of over 100 carats are not unknown. Large fine star sapphires are also found at Mogok, in addition to star rubies. Near Kabaing, at Kin, is located a mine famous for star sapphires.

The sapphires of Burma occur in intimate association with rubies in virtually all alluvial deposits throughout the Mogok area, but are found in quantity at only a few localities, particularly 8 miles (13 km) west of Mogok, near Kathé (Kathe) (Halford-Watkins, 1935b). At Kyaungdwin, near Kathé, in 1926 a small pocket was discovered that yielded "many thousand pounds' [sterling] worth of magnificent sapphires within a few weeks." (Halford-Watkins, 1935b)

Burma ruby, Mogok, ruby, sapphire, corundum, gems, gemology

Figure 41. Map of the sapphire-producing regions of Burma's Mogok Stone Tract. (Modified from Halford-Watkins, 1935b)

According to Halford-Watkins (1935b), the majority of fine sapphires were derived from the area between Ingaung and Gwebin, and the present author (RWH) also found this to be the case during his 1996 Mogok visits. Today, important mines are located at Thurein Taung and Yadanar Kaday Kadar.

One magnificent Gwebin gem mined in 1929 was scratched up just below the grass by miners preparing a site for digging. It was a water-worn, doubly-truncated pyramid weighing in at an incredible 959 ct, and was named the Gem of the Jungle. Purchased and cut by Albert Ramsay, it produced nine fine stones, ranging in size, from 66 to 4 carats (see ).

Sapphires have also been found near Bernardmyo:17

Bernardmyo itself at one time produced large quantities of sapphires, many of which were of magnificent colour and quality, though a number were of a peculiar indigo shade, which appeared either very dark or an objectionable greenish tint by artificial light. During an extensive native mining rush to Bernardmyo in 1913 a number of these stones were placed on the London market.

Many of the stones found in this area were coated with a thin skin of almost opaque indigo colour which, on being ground off, revealed a centre sometimes of a fine gem quality, but in many cases of greenish shade. The method of occurrence was different from that anywhere else as the majority of stones were taken from a hard black iron-cemented conglomerate, which was found layers a few inches thick, often only a few feet below the surface. This area now appears to be exhausted, and little mining is carried on there to-day except for peridots, which are abundant.

Another isolated local deposit which has produced some fine sapphires occurs at Chaungyi, four miles north of Mogok, and about a thousand feet higher.

J.F. Halford-Watkins, 1935b

Other than blue, sapphires also occur in violet, purple, colorless and yellow colors at Mogok. The violet and purple stones may be fine; yellows tend to be on the light side and are not common. Green sapphires are known, but rare.

Burma ruby, Mogok, ruby, sapphire, corundum, gems, gemology

Figure 42. 21.09 caratsof Burmese midnight-blue mystery. This stone, an example of Mogok's finest product, was offered in the late 1980s in Bangkok for $10,000/ct. wholesale. (Photo: Adisorn Studio, Bangkok)

Burmese sapphires compared

Although it is rubies for which Burma is famous, some of the world's finest blue sapphires are also mined in the Mogok area. Today the world gem trade recognizes the quality of Burmese sapphires, but this was not always the case. Edwin Streeter (1892) described Burmese sapphires as being overly dark. Unfortunately this error was later repeated by Max Bauer and others. G. Herbert Smith wrote…

While the Burma ruby is famed throughout the world as the finest of its kind the Burma sapphire has been ignominiously, but unjustly, dismissed as of poor quality. In actual fact nowhere in the world are such superb sapphires produced as in Burma.

G.F. Herbert Smith, 1972, Gemstones

While this statement must be qualified by adding that the finest Kashmir sapphires are in a class by themselves, those from Burma are also magnificent. J. Coggin Brown said this:

It has been stated that Burmese sapphires as a whole are usually too dark for general approval, but this is quite incorrect; next to the Kashmir sapphires they are unsurpassed. Speaking generally, Ceylon sapphires are too light and Siamese sapphires too dark, and it is more than probable that many of the best `Ceylon' stones first saw the light of day from the mountainsides of the Mogok Stone Tract.

J. Coggin Brown & A.K. Dey, 1955, India's Mineral Wealth

Not all Burma sapphires are deep in color. The best display a rich, intense, slightly violetish blue, but some are quite light, similar to those from Sri Lanka. The key difference between Burma and Ceylon sapphires is saturation, with those from Burma possessing much more color in the stone. Color banding, so prominent in Ceylon stones, may be entirely absent in Burma sapphires.

Burma ruby, Mogok, ruby, sapphire, corundum, gems, gemology

Figure 43. An offerring of small sapphires in a Mogok temple. (Author's photo; April, 1996)

Features of Mogok (Burma) sapphire

In certain respects, the inclusions in Mogok sapphires differ from their red relatives. These differences can be accounted for by the different modes of origin for each. Although mined in close proximity to one another, the sapphires are believed to have originated in pegmatites and nepheline-corundum syenites, while the rubies formed in a metamorphosed crystalline limestone.

Like the rubies, Mogok sapphires contain dense clouds of rutile silk, and a number of fine star sapphires in various shades of blue have been unearthed. Included crystals, however, are less common in the blue gems than the red, while secondary fluid inclusions are far more abundant. Finally, the color of Mogok sapphires is exceptionally even, and banding is not found in some specimens, even under immersion. The lack of sharp zoning (and presence of rhombohedral glide twinning) helps to separate Mogok sapphires from those of Sri Lanka, where it is less common.

Varieties/phenomena

  • Near colorless to rich, deep blue almost verging on the violet. Despite the stereotypical 'intense blue' Burma sapphire, many Burmese sapphires are quite light in color, wholly resembling those from Sri Lanka. The blue color of Burmese sapphires is often just slightly more violet than those of Sri Lanka.
  • Purple to violet.
  • Yellow, generally a light, straw yellow.
  • Green has been reported (U Hla Win, personal comm., 1994), but is relatively rare.
  • Six-rayed stars are common in many colors; 12-rayed stars are rare.

Occurence

Burmese sapphires have been found in a variety of environments, including pegmatites, corundum syenites, gneisses and urtites. Gems are recovered from both primary and secondary deposits.

Solids

With the exception of exsolved minerals, solid inclusions are somewhat rare in the sapphires from Mogok. Zircon has been identified as rounded grains, both with and without halos, as well as magnetite (spinel group) octahedra, large single rutile prisms and pyrrhotite (magnetic pyrite) crystals. One specimen examined by the author possessed a highly corroded tabular crystal of low relief with a pale green color. This might possibly have been olivine. Other crystal inclusions reported are apatite, monazite, fergusonite and phlogopite mica.

Burma ruby, Mogok, ruby, sapphire, corundum, gems, gemology Burma ruby, Mogok, ruby, sapphire, corundum, gems, gemology
Figure 44. Two views of a secret…
Two different looks at unknown red crystal inclusions in a Burmese sapphire from the Mogok area. (Photos by the author)

Cavities

Negative crystals are common in sapphires from the Mogok Stone Tract, although most appear to be of secondary, rather than primary, origin. Healing fissures, in all their glory, are usually profusely distributed across the stones. These range from fingerprints with slender, worm-shaped fluid channels, to curving concentrations of angular negative crystals, some two phase in nature. At times, fluid-filled fingerprints are superimposed upon these arrangements of negative crystals, suggesting two separate stages of fracturing and healing. Characteristic are the fingerprint patterns which appear folded or crumpled like flags in the wind.

Growth zoning

The color distribution of Mogok sapphires is exceptionally even; this is one of the key differences between Mogok and Sri Lankan blue sapphires. In gems from each locality the blue hue is equally fine, but one can never get too much of a good thing and Sri Lankan stones normally contain substantial areas without color. Thus, the even coloration of Mogok sapphires gives them an intensity lacking in most Sri Lankan stones. So well dispersed is the color in the former that, in many cases, even close scrutiny while immersed in di-iodomethane fails to yield evidence of the zonal banding. In the author's experience, only with the small blue sapphires from Yogo Gulch, Montana and those from Mogok is the banding often lacking.

Twin development

Polysynthetic twinning along the rhombohedron faces is common in Burma sapphires. Often accompanying these lamellae are long white boehmite needles. Such twinning is comparatively rare in Sri Lankan stones.

Burma ruby, Mogok, ruby, sapphire, corundum, gems, gemology

Figure 45. Although Burmese sapphires share a number of similar features with their cousins from Sri Lanka, polysynthetic twinning is generally not one. The rhombohedral twinning in the Mogok sapphire above is rather rare in Sri Lankan sapphires. (Photo by the author).

Exsolved solids

Rutile silk in Mogok sapphires is similar to that of the Mogok rubies. Compared with Sri Lankan stones, the silk tends to be shorter and more densely packed, and can be recognized by its spike or dart shapes. These needles lie in the basal plane and run parallel to the faces of the second-order hexagonal prism, intersecting at 60/120° angles.

The author has observed in certain Mogok sapphires what appears to be a second type of silk, differing from the rutile silk in several respects. Its color tends to be more brownish or yellowish than the rutile. Although it is oriented along three directions at 60/120° angles in the basal plane, these directions are offset 30° from that of the rutile, running parallel to the faces of the first-order hexagonal prism, not the second-order. Differences in shape are also apparent, with the new silk occurring as ultra-thin elongated plates of a distorted hexagonal outline. Possible identities include hematite, ilmenite, or a hematite/ilmenite intermixture, such as has been identified in Thai, Australian and Umba sapphires. Rarely, 12-rayed star sapphires have been found in Mogok. These possibly result from near-equal presence of both rutile and a second type of silk, as described above.

Again, like Mogok rubies, zoned clouds of minute exsolved particles are common in Mogok sapphires. While it seems possible that, in some cases, they are merely smaller versions of rutile silk, in others, differences between the silk and the particles can be seen. At times, the particles produce a pinkish reflection with overhead fiber-optic lighting. In others, the reflection is simply white. Apparently, like the two types of silk, there exist at least two types of exsolved particles in Mogok sapphires.

Exsolved boehmite needles are common. They differ radically from the orientation of the exsolved rutile silk, lying not in the basal plane, but instead along the rhombohedron faces, at the junctions of crossing twin planes. Their angles of intersection are 86.1/93.9°, as they follow the edges of the rhombohedron faces.

Burma ruby, Mogok, ruby, sapphire, corundum, gems, gemology

Figure 46. Rutile silk in a Burmese sapphire from the Mogok region. (Photo by the author)

Properties of Mogok (Burma) sapphire

Property

Description

Color range/
phenomena

  • Near colorless to rich, deep blue almost verging on the violet. Despite the stereotypical `intense blue' Burma sapphire, many Burmese sapphires are quite light in color, wholly resembling those from Sri Lanka. The blue color of Burmese sapphires is often just slightly more violet than those of Sri Lanka.
  • Purple to violet.
  • Yellow, generally a light, straw yellow.
  • Green has been reported (U Hla Win, personal comm., 1994), but is relatively rare.
  • Six-rayed stars are common in many colors; 12-rayed stars are rare.

Geologic
formation

  • Burmese sapphires have been found in a variety of environments, including pegmatites, corundum syenites, gneisses and urtites. Gems are recovered from both primary and secondary deposits.

Crystal habit

  • Unlike sapphires from most other sources, Burmese blue sapphire crystals tend to be rather tabular, consisting of short prism/pyramids with large pinacoid faces. The result is cut stones which are often flat.

RI &
birefringence

n omega = 1.757–1.765; n epsilon = 1.766–1.774 Bire. = 0.008–0.009

Specific gravity

~3.95–4.10 (higher readings in darker stones)

Spectra

Visible: Weak to strong Fe spectrum.

Fluorescence

Generally inert (LW & SW). Cr-bearing stones may show a weak red under LW.

Other features

To the best of the author's knowledge, Burmese blue sapphires are not typically heat treated. This is not for lack of trying, but because the treatment secrets of this gem have yet to be unlocked. But give them time…

Inclusion types

Description

Solids

  • Apatite (Gübelin, 1973)
  • Brookite, yellow crystals (Gübelin & Koivula, 1986)
  • Dolomite (Gübelin & Koivula, 1986)
  • Fergusonite (Gübelin, 1973)
  • Monazite (Gübelin, 1973)
  • Mica (phlogopite) (Gübelin, 1973)
  • Pyrrhotite (rare) (Gübelin, 1973)
  • Rutile, dark red prisms (Gübelin, 1953)
  • Spinel group (magnetite) (Gübelin, 1973)
  • Unidentified green crystal
  • Zircon (Gübelin, 1973)

Cavities
(liquids/gases/solids)

  • Secondary healed fractures are quite common (unlike Mogok ruby); they take on a variety of patterns and thicknesses.
  • Fractures may be lined with reddish secondary limonite stains (Gübelin & Koivula, 1986)

Growth zoning

  • Growth zoning is not so common; occasionally broad areas of zoning are seen.

Twin development

  • Growth twins
  • Polysynthetic glide twinning on the rhombohedron

Exsolved solids

  • Rutile in dense clouds of (often, but not always) short needles, parallel to the hexagonal prism (3 directions at 60/120°) in the basal plane. Rutile is reportedly rare in yellow and green stones (U Hla Win, pers. comm., May 2, 1994).
  • Boehmite, long white needles along intersecting rhombohedral twin planes (3 directions, 2 in one plane, at 86.1 and 93.9°).

a. The above is based on the author's own extensive experience, along with published reports of Eppler (1976), Gübelin (1973), Gübelin & Koivula (1986) and Kammerling & Scarratt et al. (1994).


Notes

 

13. The Thai word for ruby, taubptim, also means pomegranate. [ return to chapter text ]
Burma ruby, Mogok, ruby, sapphire, corundum, gems, gemology14. Purer in the sense that the hue position is closer to the center of the red (relative to purple and orange). [ return to chapter text ]

15. After heat treatment, Burmese rubies may contain numerous fingerprints and feathers, a result of stress-induced fracturing and subsequent healing in the oven. [ return to chapter text ]

16. It has been suggested by some that the length of the rutile needles and density of its clouds can be useful in separating Mogok and Sri Lankan rubies, but this is a test the author would not want to rely upon. [ return to chapter text ]

17. The plateau of Bernardmyo was chosen by the first British expedition to Mogok as a suitable place for a sanitarium for British troops. It was thought the climate better suited Europeans and hoped that the place would eventually develop into the Simla of Burma. Bernardmyo was christened after the first British Chief Commissioner of Upper Burma, Sir Charles Bernard (G.S. Streeter, 1887b, 1889). It was once home to the local airport, but today consists just of a small village. [ return to chapter text ]

Continued in Part 4

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