basanite

Century Dictionary and Cyclopedia

1. n. A silicious rock or jasper, of a velvety-black color, used as a touchstone for determining the amount of alloy in gold. The touchstone was formerly extensively used, but is now much less common. See touchstone and touch-needle.

Wiktionary

1. n. geology A basaltic extrusive rock, similar to chert

GNU Webster's 1913

1. n. (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.