The chemist has imitated nature in the production of colored gems. The base of these imitations, called “pastes,” is “_strass_”--a white glass compound of 300 parts of pure sand, 96 of potash, 27 of borax, 514 of white lead, and one of arsenic. The mixture is put into a crucible and kept at a high heat for 24 hours. This is the philosopher’s stone which competes with Golconda. The uncolored glass is used in making mock diamonds and white topaz. Another paste which has very great brilliancy, and, unfortunately, the same gravity as the diamond, is made by melting 100 parts of pure sand, 150 of red lead, 30 of calcined potash, 10 of calcined borax and one of arsenic, keeping the mixture melted for two or three days and then cooling very slowly. Each ingredient is separately reduced to a fine powder. FALSE RUBY is made by fusing together of strass one ounce and six drams, glass of antimony 37 grains, and purple of cassius one grain; then add eight parts more of strass and fuse for thirty hours; cool and remelt pieces in a blow-pipe. Or, melt five ounces of strass and one dram of manganese. FALSE TOPAZ can be made from 1008 grains of strass, 43 grains of glass of antimony and one grain of purple of cassius. FALSE SAPPHIRE.--Add to eight ounces of strass 52 grains of pure oxide of cobalt. FALSE EMERALD.--To one pound of strass add one dram of verdigris and fifteen grains of crocus martis. Or, take 2304 grains of strass, 21 grains of green oxide of copper, and one grain of oxide of chrome. Or, take an ounce and a half of rock-crystal, six drams of dry soda, two drams of dry borax, two drams of red lead, one dram of nitre, twenty grains of red oxide of iron, and ten grains of green carbonate of copper. FALSE CARNELIAN.--Strass two pounds, glass of antimony one pound, rouge two ounces, manganese one dram. FALSE AMETHYSTS AND OPALS are manufactured; but the fine opal defies imitation, and the amethyst is too common in nature to allow much margin for the “pastes.” In distinguishing true and false gems, no one character should be depended upon. All genuine stones will bear rough handling; if the merchant says “hands off,” refuse to purchase. Any gem worth buying is worth testing. First: try the _hardness_. The file will make no impression on the diamond and ruby, and will with difficulty scratch the other gems; while the “pastes” are easily marred. All the precious stones scratch window glass, although opal will not attack common bottle glass. All imitations easily yield to sand. The sapphire is the hardest of colored gems, and opal is the softest. The emerald will hardly scratch rock-crystal; its counterfeit not at all. Topaz will scratch ordinary ruby, but will not touch sapphire.