The turquoise which has lately been found in Arizona is not a crystal. The blue color which distinguishes it is derived from copper. It is a phosphate of alumina with water in composition. In form it is kidney shaped or stalactitic. Lazulite, a far less valuable substance, is also blue, but as it crystallizes in the monoclinic system it should not be mistaken for turquoise. Moreover, lazulite is softer and contains magnesia and lime, which the turquoise does not. Lapis lazuli, which is also occasionally mistaken for turquoise, belongs to the regular or isometric system; it is commonly massive or compact, and is a silicate of alumina with some lime and iron. It is found in syenite, crystalline, limestone, and often associated with pyrites and mica. Topaz belongs to the orthorhombic system. It is a silicate of alumina with fluorine. Powdered, mixed, and heated with microcosmic salt in the open tube, fluorine is disengaged with its characteristic odor, and etching action upon glass. With the blow pipe on charcoal, heated with the cobalt solution, it gives the fine blue color of alumina. The explorer who comes upon any hard, brightly colored stone, that may possibly turn out a gem, should preserve it carefully until he returns to some city, when it should be submitted to an expert. The value of a gem depends upon so many qualities that it were hopeless for the tyro to endeavor to arrive at any just estimate of it. He might ruin a superb specimen, without becoming one bit the wiser. A few of the more prominent characters of valuable gems follow: Name. Sp. Gravity. Hardness. Color. Aquamarine 2.7 7.7 Blue. Emerald 2.7 7.5 Green. Diamond 3.5 10.0 Colorless. Garnet 4.1 7.0 Claret. Opal 2.2 6.0 Opaline. Ruby 3.5 8.0 Dark red. Tourmaline 3.1 7.3 Various. Turquoise 2.7 6.0 Blue, green. Ultramarine 2.5 5.8 Blue to green. Graphite. This mineral is commonly known as black lead, or plumbago. It is the same in composition as the diamond, viz.: 100 per cent. carbon. Specific gravity, 2 to 2.2. Hardness, 1.2 to 1.9. Color, black. Greasy. Of value when free from impurities. Used in making pencils, stove polish, crucibles, etc. Found in the earlier rocks. Gypsum. A sulphate of lime occurring in great beds. Burnt, it becomes plaster of paris. Iron. This, the most important of all metals, is found in various forms. The ores of iron are: Sp. Gravity. Hardness. P. C. Fe. Native ore 7.7 4.5 100 Magnetite 5.1 6.0 72 Hematite 4.8 6.0 70 Limonite 3.8 5.2 60 Siderite 3.8 4.0 62 Pyrite 5.0 6.3 47