Note 1037 — BOOK XXXIII. THE NATURAL HISTORY OF METALS.647 (continued 2)

Literally “whetstone.” He is speaking of the stone known to us as Touchstone, Lydian stone, or Basanite—“a velvet-black siliceous stone or flinty jasper, used on account of its hardness and black colour for trying the purity of the precious metals. The colour left on the stone after rubbing the metal across it, indicates to the experienced eye the amount of the alloy.”—Dana, Syst. Mineral., p. 242.