basalt (n.)

type of volcanic rock, 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," also "a trial, examination, test whether anything be true," from Egyptian baban "slate," a stone which was used by the Egyptians as a touchstone of gold. According to Beekes, "It came to Greece via Lydia." In Pliny, basaniten by mistake became basalten, which is the origin of basalt.

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. From the noun in Greek came Greek basanizein "to be put to the test, be examined closely, be cross-examined, be put to torture." Not connected with salt. Related: Basaltic.