LESSON IX
HARDNESS—Continued
Minerals Used in Testing Hardness. For testing stones that are harder than a file the student should provide himself with the following set of materials:
1. A small crystal of carborundum. (Most hardware stores have specimen crystals as attractive advertisements of carborundum as an abrasive material, or the Carborundum Co., Niagara Falls, N. Y., will supply one.)
2. A small crystal of sapphire (not of gem quality, but it should be transparent and compact. A pale or colorless Montana sapphire can be had for a few cents of any mineral dealer).
3. A small true topaz crystal. (The pure white topaz of Thomas Mountain, Utah, is excellent; or white topaz from Brazil or Japan or Mexico or Colorado will do. Any mineral house can furnish small crystals for a few cents when not of specially fine crystallization.)
4. A small quartz crystal. (This may be either amethyst or quartz-topaz or the common colorless variety. The fine, sharp, colorless crystals from Herkimer County, N. Y., are excellent. These are very inexpensive.)
5. A fragment of a crystal of feldspar. (Common orthoclase feldspar, which is frequently of a brownish pink or flesh color, will do.)