gems, peridot, olivine, Pakistan

Following the Fall 1994 Gem News report on a new peridot source in Pakistan’s far western Himalayas, further study of both rough and cut stones from the deposit has been conducted. The orthorhombic peridot crystals may initially appear hexagonal due to the symmetry of “columnar” and “pyramidal” faces, with face types identified using Goldschmidt’s Atlas der Krystallformen. Prominent faces included basal pinacoid (001), columnar (010) and (110), and pyramidal (021) and (111), with striations visible along the c-axis on the (010) and (110) faces, though many crystals, as confirmed by Laura Thompson of Shades of the Earth, also exhibit well-formed, lustrous faces. Two faceted stones—a 5.23 ct rectangular mixed cut and an 18.37 ct cushion modified brilliant—showed gemological properties consistent with known peridot: medium yellowish green color, transparent diaphaneity, R.I. values of α = 1.651, β = 1.669, γ = 1.687 (birefringence 0.036), specific gravity of 3.35, weak pleochroism, inert fluorescence, and distinct absorption bands at ~453, 477, 497 nm, and a weaker one at ~529 nm. These values suggest a composition of ~90% forsterite and 10% fayalite. No inclusions were visible under magnification in the faceted stones, though black rod-like inclusions, identified as ludwigite via X-ray diffraction, were noted in some cabochon-grade rough.