basanite

from The Century Dictionary.

• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun geology A basaltic extrusive rock, similar to chert