黑曜石 Obsidian is a naturally occurring volcanic glass. It is formed when felsic lava extruded from a volcano cools without crystal growth. Obsidian has been known since prehistoric times, and has some fascinating properties as well as some important practical uses. In addition to its use as a gemstone, obsidian has been used in cardiac surgery, as well-crafted obsidian blades have a cutting edge many times sharper than high-quality steel surgical scalpels.
In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern, and this is known in the gem trade as snowflake obsidian.
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