Rock & Mineral Encyclopedia

Search and identify 1,000+ rocks, minerals, crystals, and gemstones — with properties, formation, colors, hardness, and how to tell them apart.

Devitrified Obsidian

Obsidian that has partly crystallized over time, growing pale spherulite clusters within the black glass, as in snowflake obsidian.

A rock formed from hot pyroclastic flows, often welded, sometimes containing flattened glass lenses called fiamme.

Obsidian containing spherulites — small radiating spheres of feldspar and cristobalite that crystallized within the cooling volcanic glass.

An uncommon purple to violet tourmaline colored by manganese, the deeper reddish-purple stones sometimes called siberite.

Soft, lightweight siliceous rock made of fossilized diatom shells, valued as a filter, abrasive, and absorbent.

Volcanic glass marked by parallel or swirling bands of differing color that record the flow layering of cooling lava.

Obsidian crossed by parallel flow bands of differing color, formed as layers of lava with slightly different compositions froze into glass.

A fine-grained, dark volcanic rock that erupts as fluid lava and forms most of the ocean floor and many lava plateaus.

A limestone packed with visible shells and shell fragments, recording the accumulation of marine invertebrate remains on ancient sea floors.

A fine-grained, silica-rich volcanic rock that is the extrusive equivalent of granite, often pale, banded, or flow-textured.

Black volcanic glass laced with delicate web-like veins of contrasting color, formed by flow banding and fine crystallization.

A black volcanic glass threaded with delicate grey, swirling lace-like bands of flow lines that show beautifully when polished.

A fine-grained, strongly foliated rock formed deep in fault zones where rocks flowed and ground down rather than fracturing.