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An unusual YAG with a “reverse” colour change

Authors: Gagan Choudhary and Chaman Golecha

(This article was first appeared in Gems & Gemology, Vol. 43, No. 4, pp 387 -388)

Yttrium aluminum garnet (YAG) is manufactured primarily for industrial purposes, with some “leftovers” used as a gem simulant. Though YAG was first produced commercially in colourless form, it has since been seen in numerous hues, such as green, yellow, pink, red, blue, and “lilac.” Most YAGs are eye-clean, but a few included specimens have been reported (see, e.g., Gems & Gemology, Winter 1993, Lab Notes, p. 284).

A 9.44 ct oval mixed cut gem was encountered at the Gem Testing Laboratory, Jaipur, India. Standard gemmological testing identified it as YAG. In daylight the sample appeared yellow, with some tinges of orange and it contained abundant eye-visible planes of inclusions (figure 1). The desk-model spectroscope revealed a series of strong lines and bands across the spectrum i.e., a typical rare-earth spectrum. The sample fluoresced a strong reddish orange to UV radiation.

Figure 1: This 9.44 ct YAG was unusual due to its heavily included nature and its “reverse” colour change: yellow in incandescent light (left) and daylight, and orangy pink in fluorescent light (right).

With magnification, the YAG displayed a complex array of etch channel–like inclusions (figure 2), gas bubbles (figure 3), and angular growth zoning (figure 4). The etch channel–like inclusions were highly reflective, and appeared either opaque or flux-like depending on the viewing direction and illumination. Some cloudy patches were also seen. We have not observed such inclusions in YAG in our laboratory, although somewhat similar features have been reported (see the Lab Note cited above). The presence of gas bubbles indicated that it was a product of Czochralski pulling and not a flux growth process; however, the possibility of production via the floating zone technique cannot be ruled out.

Figure 3: Numerous gas bubbles were present throughout the YAG, along with complex tube-like inclusions.

The sample became even more interesting when it was viewed with different types of illumination. In daylight-equivalent fluorescent light, it appeared orangy pink (again, see figure 1). However, when viewed in incandescent light, it appeared yellow i.e., the same as the colour seen in daylight. As such, this YAG showed a “reverse” effect, since colour-change stones generally appear pink or red in incandescent light. This type of colour change has been referred to as type 2, and is commonly seen in manufactured glasses, and, rarely, some other gems (e.g., Y. Liu and B. A. Fry, “A colourimetric study of a tourmaline from Mozambique which shows a reverse alexandrite effect”, Journal of Gemmology, Vol. 30, No. 3/4, 2006, pp. 201–206). Consistent with the YAG reported here, Liu and Fry (2006) indicated that type 2 stones show a colour change between daylight and daylight-equivalent fluorescent illumination, as well as between fluorescent and incandescent light, but not between daylight and incandescent light. The cause of the type 2 colour change in this YAG is not known.

All photographs and photomicrographs by Chaman Golecha