GEMMOLOGY OF SOME COLOURED SYNTHETIC QUARTZ

Abstract

ldentification of coloured quartz varieties and their discrimination from other minerals are not difficult for a gemmologist. However , quartz and its coloured varieties, being grown under condi-tions similar to natural crystallization environ-ment, create more problems than any other synthetic material. This is particulary true for synthetic amethysts, Fe-citrines and smoky , quartz whose structural and morphological properties and colour centres are virtually identical to those of natural crystals. To recognize synthetic quartz among faceted coloured stones a gemmologist should consider distribution of numerous inhomogeneities (growth pyramids and zones, twins, inclusions etc.) in the bulk of the crystal as well as all probable directions of cutting and various orientations of these directions in the process of faceting the stone in question. All coloured synthetic quartz material is known for the absence of chlorite, sericite, hematite, amphiboles, metal sulphides and other fluid inclusions common for natural crystals. Colour itself may be a determining factor in identification of green, brown and blue synthetic varieties. The most convincing sign for the synthetic nature of Fe-citrines is an absence of Dauphine and of polysynthetic Brazilian twins in synthetic crystals. It is relatively simple to identify synthetic amethyst stones made of crystals grown in nearly neutral fluoride solutions. They always display some "streamline" pattern (fiber structure) with orientation following crystal optical axis. Furthermore, they have no twins and their IR absorption spectra have characteristic band at 3640,3670 and 3685cm<-1>, common for all quartz grow, n in F-bearing media. These bands have never been observed in natural quartz, nor in the crystals synthesized in alkaline solutions. Amethyst synthesized in alkaline solutions is hardly distinguishable from natural amethyst. It is especially true when ston'es are in high grade with homogenous colour distribution and with-out twins. If stones are too homogeneous and contain no twins, those who are familiar with amethyst will be suspicious of the origin. The ex-istence of Da, uphi6 and Brazilian twins may offer proofs, though not conclusive, of natural origin, and simplify identification of amethysts.