194 QUARTZ FAMILY, ever, be too rough, otherwise the particles of the metal will be hid amongst inequalities, and no distinct or con tinuous trace will be formed. Lastly, a good touchstone must have a black colour, as this tint shews the colour of the streak better than any other. Those varieties of Lydian-Stone which are neither too hard nor too soft, and which have a kind of velvety feel, and are not traversed by quartz veins, are those ^ which are preferred for touchstones. They are cut into tables by means of pumice; then ground with sandstone, and, lastly, rubbed with charcoal- powder or ivory-black. Observations. > < 1. Compact varieties of Clav-slate and of Basalt are sometimes used as touchstones. 2. According to Humboldt, it contains a small por tion of carbon. 3. It was first noticed in Lydia, whence its name. It is described by Pliny and Theophrastes under its present name.