Obsidian & Variations 

Obsidian is the name given to volcanic glass – a rock rather than a mineral, it is a mixture of cryptocrystalline grains of silica minerals in a glass-like suspension and displays a conchoidal fracture. Obsidian forms during the last stage of volcanic activity, close to the earths surface cooling occurs too quickly for crystals to grow. Natural obsidian varieties include black, bluish, mahogany, gold & silver sheen, rainbow, etc. – the colors due largely to refraction by microscopic bubbles (and microscopic mineral inclusions such as magnetit in “Rainbow Obsidian”). Lustre: Vitreous