Snowflake Obsidian gemstone

Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock which has been used as a gemstone since antiquity. It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth. Obsidian was named after Obsius, a Roman who discovered a similar stone. Obsidian forms when lava cools and whilst it is mineral-like, it is not considered to be a true mineral because its composition is too complex and it does not have a crystalline structure.