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

Bona Hiu Yan Chow

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.

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.