Obsidian is a natural glass. It is formed from volcanic lava that cooled too quickly for significant crystallization to occur. Hence it is amorphous, with no cleavage; fracture is conchoidal. Obsidian is usually black, but brown, grey and more rarely red, blue and green material is found. The color may be uniform, striped, or spotted. Some inclusions give obsidian a metallic sheen, while internal bubbles or crystals (called crystallites) produce a "snowflake" effect (hence snowflake obsidian) or an iridescence seen as flashes of colour.
Obsidian is found in areas where there is, or has been, volcanic activity: for example, Hawaii (USA), Japan, and Java. Other localities include Iceland, Hungary, the Lipari Islands off Italy, the former USSR, Mexico, Ecuador, and Guatemala. Dark nodules found in Arizona and New Mexico (USA) are called "Apache tears".

Obsidian has been used since the prehistoric times for making tools, weapons, masks, mirrors, and jewellery. The very sharp shards of the natural glass have been fashioned as blades, arrowheads, and daggers. Today most obsidian jewellery comes from North and Central America.

1.	Chemical Composition	Varies, but 66% - 72% is silica
2.	Colors / Varieties	Color: Black, grey, dark brown Transparency: Transparent to opaque. Varieties: Rainbow obsidian with iridescent sheen; Flowering obsidian is black with white spherulitic inclusions.
3.	Hardness	5
4.	Specific Gravity	2.33 - 2.42
5.	Optic Character	Isotropic, S.R., A.D.R. (S.R.)
6.	Refractive Index / Birefringence	1.48 - 1.52 / Nil.
7.	Magnification	Gas bubbles, crystallite inclusions which are round, rod-shaped or coiled, fine silky striations (cat's eye effect), color zoning and banding.
8.	Geological Occurrence	Formed by the rapid cooling of volcanic lava.
9.	Sources	Siberia, Mexico, Hungary, Hawaii, Japan, Iceland, Italy, U.S.A. (Nevada, Arizona - Yellowstone Park)