Obsidian

Formed from the quick cooling of lava, obsidian is not a true mineral as it is glass, rather it’s called a mineraloid.  It freezes along the fast cooling edges of lava flows or when lava meets water.  It is often black but can be multicolored with swirls of red brown or green.  You can learn more about it in this Transmission from Devin at EveryDay Earth Headquarters:

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