Obsidian

What it actually is

A volcanic glass, typically around seventy per cent silica, amorphous, hardness 5 to 5.5, with conchoidal fracture.

Because it has no crystal structure, obsidian breaks along smooth curved surfaces, and a fracture edge can be a single molecule thick at its finest, sharper than a surgical steel scalpel. Obsidian blades have been used in experimental surgery for exactly this reason.

Colour is usually black from magnetite and hematite inclusions. Snowflake obsidian contains cristobalite spherulites; mahogany obsidian contains iron oxide bands; sheen and rainbow obsidian contain oriented mineral inclusions or bubble layers producing an optical effect.

Obsidian is metastable. Over geological time it devitrifies, crystallising slowly, which is why very little obsidian older than a few million years survives as glass.

It is chemically inert and safe to handle, but knapped edges cut skin easily.

Where it comes from

Mexico, particularly the Pachuca and Otumba flows, supplied Mesoamerica. Turkey's Cappadocia and the Aegean island of Melos supplied the ancient Near East and Mediterranean. Armenia, Iceland, Italy's Lipari, Japan and the western United States all produce it.

The archaeological importance is exceptional. Each volcanic flow has a distinctive trace-element signature, so obsidian artefacts can be matched to their source flow by geochemical analysis. That has allowed archaeologists to map prehistoric trade networks in detail unavailable for almost any other material.

Melian obsidian found in mainland Greek sites dating to around 11000 BCE is among the earliest evidence of seafaring in the Mediterranean.

In Mesoamerica obsidian was traded, taxed and worked at industrial scale, and the Aztec macuahuitl, a wooden club edged with obsidian blades, was a principal weapon.

What the traditions claimed

The Mesoamerican tradition is the substantial one. Polished obsidian mirrors were the instrument of Tezcatlipoca, whose name means smoking mirror, and Aztec priests used them for divination. The Spanish recorded the practice, and one such mirror later belonged to John Dee, the Elizabethan scholar and occultist, and sits in the British Museum today.

That single object traces a route from Aztec ritual to Renaissance English magic, which is a remarkable survival.

In Anatolia, obsidian mirrors from Catalhoyuk date to around 6000 BCE and were placed in burials.

Classical sources describe obsidian and Pliny attributes its name to a Roman named Obsius or Obsidius who reportedly brought it from Ethiopia.

Modern practice reads obsidian as a truth stone and a protective one, and it is frequently described as confronting rather than comforting. That reading is twentieth-century but it fits the object well.

How it is worked with

Obsidian's modern brief is unusual and worth respecting: it is described as a stone that shows you what you have been avoiding, and practitioners generally treat it as uncomfortable by design.

That is not a beginner's exercise and it is not the right object when what you need is reassurance.

The scrying use has a real historical basis, and a polished obsidian mirror is the most authentic scrying surface available. The practice is simply sustained gazing at a dark reflective surface until the mind supplies images, and the tradition around it is well documented.

For carrying, a tumbled piece is safe. Raw knapped material has genuinely sharp edges and should be handled and stored carefully, particularly around children.

At hardness 5.5 it scratches and chips, so it is not a working ring stone.

Buying it honestly

Black glass is sold as obsidian and is chemically not far off, since obsidian is glass. The difference is origin rather than composition.

Look at a thin edge in strong light: real obsidian usually transmits a faint brown, green or grey, while cheap manufactured glass is often dead black. Manufactured pieces show mould seams and perfectly regular round bubbles; obsidian shows elongated or irregular bubbles and conchoidal ripples.

Snowflake, mahogany and rainbow obsidian are natural varieties. Bright blue, green and purple obsidian is manufactured glass, since natural obsidian does not occur in those colours.

Sheen obsidian is genuine but easily imitated.

Moulded obsidian pyramids, spheres and skulls are usually carved rather than moulded, and a seam is a warning sign.