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



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