Silicates

Aquamarine, emerald, tourmaline, topaz, zircon, peridot, spodumene, and garnet are gem minerals known by the general name "silicates" and contain both silicon and oxygen as their major constituents. Ruby, sapphire, chryso-beryl, and spinel are "oxides," containing oxygen as a major constituent. As already mentioned, each kind of atom is limited in the kind and number of other atoms it can join. The chemist that are also making diamond engagement rings, through the years, has learned to predict the possible combinations and has developed very accurate methods of checking them in the laboratory. He can make almost innumerable combinations of elements in the laboratory, predicting in each case how they will combine with each other. He can also determine the kinds and relative quantities of atoms present in a mineral sample and use a standard method of noting them. His typical analysis of a mineral might show that there are equal numbers of zirconium (Zr) and silicon (Si) atoms present and four times as many oxygen (O) atoms. His notation, then, would read ZrSi04. This is the chemical notation or formula for the gem mineral zircon. Thus, the formula for aquamarine is Be3Al2Si6Oi8—a beryllium aluminum silicate; for chrysoberyl it is BeAl204—beryllium aluminum oxide.

All this seems simple enough until it develops that the chemical formulas for ruby and sapphire are identical—A1203. If absolutely pure, this aluminum oxide, A1203, is colorless. Ruby, however, is red, and sapphire by definition is any color except red. As the formula indicates, they are actually the same mineral, but ruby is aluminum oxide containing very small traces of the element chromium which cause it to have the red color. Sapphire seems to get its colors from tiny traces of iron or titanium, or both together. Certainly, this is a case where chemical impurities gathered by a mineral during its formation produce highly desirable results.