SNOWFLAKE OBSIDIAN
Obsidian is unstable on geologic time scales.
Eventually, obsidian will convert on its own to a finely-crystalline
mass. A partially-converted obsidian is an attractive rock called snowflake
obsidian. The black portions of the rock below are rhyolitic obsidian
(glass). The white patches are devitrification spots composed of
cristobalite (SiO2, a polymorph of quartz).
Snowflake obsidian