Lab Notes Gems & Gemology, Summer 2018, Vol. 54, No. 2

A Cat’s-Eye Rock Consisting of Jadeite and Amphibole

Chatoyancy on one side of green bead.
Figure 1. This 1.42 ct translucent variegated green bead displays a distinct chatoyant band on the darker green side (left). No chatoyancy was observed on the lighter green side (right). Photos by Tony Leung.

The Hong Kong laboratory recently examined a translucent variegated green bead exhibiting chatoyancy (figure 1). The 1.42 ct bead, measuring approximately 5.62 mm in diameter, displayed two different shades of green on opposite sides. A distinct cat’s-eye band was readily visible across the darker green region with fiber-optic illumination, whereas no chatoyancy was observed in the lighter green region on the reverse side.

Inclusions in the green bead.
Figure 2. Left: Parallel fibrous amphibole crystals were the cause of chatoyancy in this green bead; field of view 11.34 mm. Right: Tight interlocking grains were visible in the lighter green jadeite region; field of view 3.80 mm. Both observed with fiber-optic illumination. Photomicrographs by Bona Hiu Yan Chow.

Microscopic observation revealed that the darker green side consisted of parallel fibrous crystals (figure 2, left). Light reflecting off these oriented crystals produced a chatoyant band perpendicular to the length of the crystals. On the lighter green side, tight interlocking grains were observed (figure 2, right). Raman spectroscopy detected the presence of richterite, an amphibole with the general formula Na2CaMg5Si8O22(OH)2, in the darker green region and jadeite in the lighter green region.

While it is not unusual to encounter a rock consisting of jadeite and amphibole (G. Shi, “Mineralogy of jadeitite and related rocks from Myanmar: a review with new data,” European Journal of Mineralogy, Vol. 24, No. 2, 2012, pp. 345–370), it is fascinating to see such a rock being fashioned into an attractive variegated green bead exhibiting chatoyancy.

Bona Hiu Yan Chow is a staff gemologist at GIA in Hong Kong.