Green Aventurine

The optical phenomenon that defines aventurine — that glittering, shifting inner light — has its own name in mineralogy: aventurescence. It is caused by the reflection of light from a dense population of flat, highly reflective mineral platelets oriented more or less parallel within the host material. In green aventurine, those platelets are fuchsite, a chromium-substituted variety of muscovite mica; the chromium is also responsible for the green colour, making fuchsite simultaneously the pigment and the glitter in the stone. The more densely packed and well-aligned the mica platelets, the more dramatic the shimmer — high-quality Indian aventurine from Rajasthan, where the stone has been quarried and worked for centuries, can show an almost metallic sheen that is quite arresting. Lower-density material looks flatter, more like a simple green chalcedony, with only occasional points of light.

It is worth knowing that aventurescence is not exclusive to quartz. The same effect appears in sunstone (feldspar with copper or hematite platelets producing an orange-gold shimmer), in goldstone (actually a man-made glass with copper crystals — sometimes sold misleadingly as natural), and in a handful of other minerals. The name aventurine itself is said to derive from the Italian a ventura, meaning “by luck” or “by chance,” after the accidental discovery of aventurine glass by Venetian glassmakers in the 18th century — though the natural stone predates that name considerably. The Indian material, shaped into beads and carvings, has been traded along ancient routes since well before written accounts catch up with it. It is, in every sense, a stone that seems to accumulate good history.