Obsidian

Obsidian is a natural volcanic glass, formed when viscous, silica-rich (rhyolitic) lava cools so fast that crystals have no time to grow. Usually glossy jet-black, it breaks with a smooth conchoidal fracture into razor-sharp edges — a property that made it a prized material for tools and blades throughout human prehistory.

Identifying obsidian

Obsidian is a glass, not a mineral, so it has no crystal structure; it is moderately hard (about 5–5.5) with a bright glassy lustre and curved, shell-like fracture. Attractive varieties include snowflake obsidian (with grey-white cristobalite 'snowflakes'), mahogany obsidian (brown and black) and rainbow or sheen obsidian (iridescent from tiny inclusions).

Where it is found

Obsidian occurs around young volcanoes worldwide — the western United States, Mexico, Iceland, Italy (Lipari), Turkey and many other volcanic regions.

For collectors

Clean black glass, well-developed snowflake patterning and strong rainbow sheen are the most sought-after; knapped pieces and natural 'Apache tears' nodules are also popular.