The West Coast laboratory recently studied a most unusual heat-treated sapphire. Examination of the 1.04 ct stone with a gemological microscope and a fiber-optic illuminator not only showed evidence typical of heat treatment—such as diffused color zoning, ruptured inclusions, and pits with heat-damaged surfaces—but it also revealed parting planes that were decorated with numerous dendrites of an unknown light-to-dark green material (see figure). The grayish green color and transparency were observed only in the larger dendrites. The dendrites showed no pleochroism or birefringence, which suggested that they were singly refractive. 

Some of the dendrites had been polished on edge and exposed on the surface during faceting. Laser Raman microspectrometry of one of these very small exposed surfaces revealed that the dendrites were spinel.

This immediately brought to mind the discovery and subsequent analysis in 1989 by Dr. Henry Hänni, director of the SSEF Swiss Gemmological Institute (Basel, Switzerland), of dendritic spinel inclusions in association with a glass component in a heat-treated ruby (“Behandelte Korunde mit glasartigen Füllungen,” Zeitschrift der Deutschen Gemmologischen Gesellschaft, Vol. 35, No. 3/4, 1986, pp. 87–96). Dr. Hänni concluded that these inclusions had resulted from the treatment process. On the basis of that work and our recent discovery of dendritic spinel inclusions in a heat-treated blue sapphire, it appears that we can expect to see these features occasionally in both rubies and sapphires. Since such spinel dendrites are rarely encountered, however, it is important to know of their existence and especially to be able to recognize them as products of heat treatment. They should not be mistaken for, or interpreted as, natural inclusions.