Obsidian

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Obsidian is a naturally occurring volcanic glass formed during the eruption of felsic lavas, distinguished by having high concentrations of the chemical element silica. With their high silica content, felsic lavas do not behave like the mafic, or silica-poor, lavas found on the island of Hawaii.

Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline; in addition, its composition is too complex to comprise a single mineral. It is sometimes classified as a mineraloid. Though obsidian is usually dark in color similar to mafic rocks such as basalt, obsidian’s composition is extremely felsic. In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern (snowflake obsidian). A rainbow-like sheen (rainbow obsidian) is caused by inclusions of magnetite nanoparticles. Obsidian may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. These bubbles can produce interesting effects such as a golden sheen (sheen obsidian).