Lydian stone, or black jasper, a variety of siliceous or flinty slate, of a grayish or bluish black color. It is employed to try the purity of gold, the total amount of alloy becoming indicated by the shade left on the stone when rubbed because of the steel.
A silicious rock or jasper, of a velvety-black shade, made use of as a touchstone for identifying the quantity of alloy in silver. The touchstone was formerly thoroughly utilized, it is now significantly less common. See touchstone and touch-needle.