A doublet, being usually constructed of a garnet top and a glass back, may resist a file at the girdle if the garnet top covers the stone to the girdle, as is sometimes the case, especially in the smaller sizes. In this case the back must be tested. One should never pass a file rudely across the corners or edges of the facets on any stone that may be genuine, as such treatment really amounts to a series of light hammer blows, and the brittleness of most gem stones would cause them to yield, irrespective of their hardness. It should be remembered that some genuine stones are softer than a file, so that it will not do to reject as worthless any material that is attacked by a file. Lapis lazuli (5), sphene (5), opal (6), moonstone (6), amazonite (6), turquoise (6), peridot (6-1/2), demantoid garnet (6-1/2) (the "olivine" of the trade), and jade (nephrite) (6-1/2), are all more or less attacked by a file. TABLE OF HARDNESS OF THE PRINCIPAL GEM-STONES 10. Diamond. 9-1/2. (Carborundum.) 9. Sapphire and ruby (also all the color varieties of sapphire). 8-1/2. Chrysoberyl (alexandrite). 8. True topaz and spinel (rubicelle, balas ruby). 7-1/2. Emerald, aquamarine, beryl, Morganite, zircon (jacinth and true hyacinth and jargoon), almandine garnet. 7-1/4. Pyrope garnet (Arizona ruby, cape ruby), hessonite garnet (cinnamon stone), tourmaline (various colors vary from 7 to 7-1/2), kunzite (7+). 7. Amethyst, various quartz gems, quartz "topaz," jade (jadeite). 6-1/2. Peridot (chrysolite), demantoid garnet ("olivine"), jade (nephrite). 6. Opal, moonstone, turquoise. 5. Lapis lazuli. 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.)