Obsidian

Obsidian is lava that has cooled without forming crystals, giving it a glassy texture. Generally obsidian is said to result from very rapid cooling. But typical obsidian comes from a dry magma that is so very high in silica, and hence so viscous (like its relative rhyolite), that it surely can sit around for a long time, like thousands of years, without crystallizing.

This obsidian lump comes from California's Napa Valley, where volcanic deposits help create the rich soil there. The outer rind shows signs of hydration from being buried in the soil for thousands of years. The thickness of this hydration rind is used for telling the age of obsidian, and hence the eruption that produced it.

Note the faint bands on the outer surface. These always remind me of taffy, and they result from mixing of the thick magma underground. The clean, conchoidal fracture surface shows why obsidian was valued by the native people for making arrowheads and other tools. Chunks of obsidian are found far from their place of origin because of prehistoric trading, and therefore they bear cultural as well as geologic information.

For photos of related rocks, see the extrusive volcanic rocks gallery.

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