Surface ('bulk') diffusion-treated corundums (SDTCs) created a stir in the Bangkok market in 1990. This article looks at the original patents and continues through the so-called "Deep Diffusion" treatment. Identification methods are discussed, as well as the backroom politics of disclosure. At the end is a quick-reference table summarizing key identifying features of all types of natural, treated and synthetic corundums.

A discussion of brilliance, windows and extinction as they relate to the face-up appearance of colored gemstones. Diagrams demonstrate that extinction is largely a function of cut.

Probably the first opinion piece I ever wrote, it was my take on diamond grading from the standpoint of someone working with colored stones.