basanite

n. [ L. basanites lapis, Gr. ba`sanos the touchstone: cf. F. basanite. ] (Min.) Lydian stone, or black jasper, a variety of siliceous or flinty slate, of a grayish or bluish black color. It is employed to test the purity of gold, the amount of alloy being indicated by the color left on the stone when rubbed by the metal. [ 1913 Webster ]