Lab Notes Gems & Gemology, Spring 2018, Vol. 54, No. 1

Cat's-Eye Demantoid and Brown Andradite with Horsetail Inclusions

Cat's-eye demantoid with well-aligned horsetail inclusions.
Figure 1. This 2.51 ct green demantoid with chatoyancy (left) contained well-aligned fibers of horsetail inclusions (right, field of view 1.10 mm). Photos by Shunsuke Nagai (left) and Taku Okada (right).

Two garnets were recently submitted to the Tokyo lab for identification. One was a green cabochon weighing 2.51 ct; the other was an orangy brown round brilliant weighing 2.47 ct. Both had a refractive index (RI) over the limits of the refractometer and a hydrostatic specific gravity (SG) of 3.84. To confirm the identification, we used Raman spectroscopy. Both stones matched the andradite garnet group from the reference database. The ideal chemical formula of andradite is Ca3Fe2Si3O12. The most important feature in both garnets was their horsetail inclusions. The presence of horsetail inclusions, which are thought to consist of chrysotile fibers branching out and radiating toward the surface from a small chromite nodule, has been regarded as diagnostic for natural demantoid.

The 2.51 ct stone was a rare cat’s-eye demantoid (figure 1). Demantoid garnet is by definition the green-colored andradite caused by chromium substitution. Chatoyancy is usually caused by closely spaced bands of long, thin parallel needles or fibers in the correct orientation, with a polished rounded surface such as a cabochon to reflect the light off the parallel inclusions. In this stone, the chatoyancy was due to the fibers of the horsetail inclusions. The radiating fibers were not parallel straight lines but parallel curves, so the “eye” was slightly curved.

Orangy brown andradite with horsetail inclusions.
Figure 2. This 2.47 ct orangy brown andradite (left) contained radiating horsetail inclusions regarded as exclusive to demantoid (right, field of view 1.60 mm). Photos by Shunsuke Nagai (left) and Taku Okada (right).

The 2.47 ct stone was an andradite garnet with classic horsetails (figure 2). The dominant bodycolor was brown. The gemological properties and chemistry were classified as andradite in the garnet solid solution, but it was poorer in calcium and richer in iron and silicon than the ideal andradite composition. The brownish cast was due to iron. On occasion we encounter stones with a strong yellow component that still have enough green to qualify as demantoid. However, this stone had no green in it and therefore was just andradite, not demantoid.

Consequently, the horsetail inclusions cannot be considered exclusively diagnostic for demantoid. They may also be found in non-green andradite. These two identifications—“cat’s-eye demantoid” and normal “andradite” in spite of horsetail inclusions—were issued for the first time in GIA’s colored stone report records.

Taku Okada is a staff gemologist at GIA in Tokyo. Philip G. York is a senior staff gemologist at GIA in Carlsbad, California.