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obsidian

ob·sid·i·an [ əb síddee ən, ob síddee ən ] (plural ob·sid·i·ans)


noun 
Definition:
 
jet-black volcanic glass: a jet-black volcanic glass, chemically similar to granite and formed by the rapid cooling of molten lava, that was used by early civilizations for manufacturing tools and ceremonial objects

[14th century. < Latin (lapis) Obsidianus, copyist's error for Obsianus "(stone) of Obsius," a Roman]
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