Obsidian

How a rock can be a glass

Cooling rate, again, decides everything. Basalt erupting into air loses heat fast but still manages crystals under a millimetre. Rhyolitic magma with a high silica content is far more viscous and conducts heat slowly, so a thin flow or a lava dome margin can drop through the crystallisation window in hours. The melt's atoms lock into place without ordering, and the rock that results behaves like a supercooled liquid rather than a crystalline solid. That is why obsidian shows a conchoidal fracture: there are no planes of weakness to follow, so a crack propagates in a smooth curved sheet.

Recognising it in three moves

Obsidian is one of the few rocks you can identify from across a stream, but the field checks are worth doing properly.

• Look for the fracture. Circular ripple marks radiating from an impact point are visible on almost every fresh surface.
• Check the translucency. Hold a flake edge to the sun; under a millimetre thick, obsidian goes smoky grey-brown.
• Confirm there is no granularity. Even at 10× magnification the surface is smooth — no green specks, no white laths, nothing.

Banding, flow and the colour variants

Rarely is obsidian uniform black. Flow banding — alternating black and brown-grey streaks — records the shearing of viscous lava as it moved, and it is usually parallel to the old flow direction. Mahogany obsidian has red-brown iron oxide streaking; snowflake obsidian contains white radial clusters of cristobalite that grow during slow devitrification; rainbow obsidian owes its sheen to nanometre-scale layers trapping thin-film interference, the same effect that colours a soap bubble. All of these are the same glass with different impurities and cooling histories.

The devitrification clock

Obsidian is metastable. Left alone, glass slowly reorders into tiny crystals — feldspar, cristobalite, quartz — a process called devitrification, and the grey speckled rims on old specimens show it happening. Perlitisation follows, when hydration fractures the glass into onion-like concentric shells. Both processes are time- and temperature-dependent, which is why obsidian from a 20-million-year-old field is grey and dull, while a Hawaiian flow a few centuries old is still jet black and glassy. If you are collecting, this is the feature to record: rim thickness and the proportion of grey devitrified material give you a rough relative age at a glance.

Where it survives in the record

Obsidian does not last. It is chemically reactive, hydrates steadily, and turns into clay in wet tropical climates within a few hundred thousand years. That is why obsidian sources are almost always geologically young — Quaternary or late Tertiary — and why an obsidian artefact found far from any volcano is evidence of trade, not of local eruption. Lipari glass travelled across Neolithic Europe; Yellowstone obsidian moved across the Great Plains. As a field collector you should note the locality precisely, because chemically fingerprinting a piece of archaeological obsidian starts with knowing which flows were available within walking distance.

Sources

Hardness, streak, cleavage and specific-gravity values quoted on this sheet are checked against the references below.

• Geology.com Rocks — rock-type reference — descriptions and photographs of igneous, sedimentary and metamorphic rocks
• British Geological Survey — UK national geological survey — the three rock types, the rock cycle and mineral definitions