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Compounds of metals and oxygen are called oxides, and metallic salts of carbonic acid are called carbonates, e.g. Iceland Spar. In like manner, hydro-fluoric acid gives us fluorides, e.g. fluor spar, phosphoric acid, phosphates, e.g. turquoise, silicic acid, silicates, e.g. emerald, sulphuric acid, sulphates, boracic acid, borates, titanic acid, titanates, chromic acid, chromates, and hydrochloric acid gives us chlorides.
But if all gem stones were analyzed, it would be found that the following elements would be the most conspicuous:
Silicon, aluminum, iron, carbon, oxygen, hydrogen, copper, calcium, magnesium, sodium, manganese, chromium, potassium, phosphorus, sulphur, zinc, zirconium, and beryllium. Silicon does not occur in a free state in nature, but usually in the form of silica, which is a combination of silicon and oxygen. In this form, it constitutes a considerable portion of the earth’s crust—some Go per cent—and many well known varieties of gem quartz are composed of this material. It figures largely also in the composi-lion of flint, sand, and all silicates.
With the exception of diamond, which is almost pure carbon, most of the other gem stones are a natural combination of two or more of the above named elements. Thus rock crystal is a combination of oxygen and silicon in certain definite proportions, ruby is a combination of aluminum and oxygen, although most gem stones are more complex in composition than these two examples. It should be noted that the color of a stone is an optical property and is due to a very minute inclusion of some foreign matter, which might be organic or inorganic, and which is rarely expressed in the chemical composition of the stone.