THE MATERIAL LIBRARY
Obsidian
Natural volcanic glass formed by rapidly cooling lava.
Colors & patterns
Usually black; mahogany, snowflake, rainbow and sheen varieties also occur.
Typical origins
Volcanic regions including Mexico and the western United States are notable sources.
Hardness
About 5–5.5
Choosing & setting this material
Its glassy polish makes bold cabs, but edges can chip. It differs from both crystalline quartz and manufactured glass.
Care & cleaning
Clean gently with lukewarm water, a little mild soap and a soft cloth, then dry. Avoid steam, ultrasonic cleaners, harsh chemicals and sudden temperature changes, especially when treatments are unknown.
Origins above describe typical sources of the material, not the origin of an illustrated or stocked stone. Hardness measures scratch resistance, not resistance to breaking. Confirm the identity and treatments of the actual piece.
Further reading: Material reference · GIA: gem treatments