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c.1600, from Late Latin basaltes, misspelling of Latin basanites "very hard stone," from Greek basanites "a species of slate used to test gold," from basanos "touchstone." Not connected with salt. Said by Pliny ["Historia," 36.58] to be an African word, perhaps Egyptian bauhan "slate." Any hard, very dark rock would do as a touchstone; the assayer compared the streak left by the alleged gold with that of real gold or baser metals. Hence Greek basanizein "to be put to the test, examined closely, cross-examined, to be put to torture."
A dark, fine-grained, igneous rock consisting mostly of plagioclase feldspar and pyroxene, and sometimes olivine. Basalt makes up most of the ocean floor and is the most common type of lava. It sometimes cools into characteristic hexagonal columns, as in the Giant's Causeway in Anterim, Northern Island. It is the fine-grained equivalent of gabbro.