The description of gem stones and their properties necessitates the use of certain terms, which we will now proceed to define. If we do not understand these terms, they will appear to be some meaningless jargon which is being constantly repeated, but actually they are only a clear and concise method of expressing certain physical and chemical properties.
For instance, we may know what a “crystal” means, or what “hardness” generally denotes, but we must be more exact when we use such words in connection with stones. The study of gemology entails the grasp of some kindred sciences, and the wider knowledge we acquire of certain branches of physics and chemistry, and more particularly crystallography, light, and the structure of matter, the more we shall understand gem stones. Yet without going into such subjects deeply, there is plenty of interest for everyone in these precious and beautiful products of nature.
With very few exceptions, all gem stones are natural minerals. They are, therefore, the product of an inorganic process and are composed of the same substance throughout. All have a definite chemical composition, which generally may be expressed by a chemical formula. In other words, all true gem stones are formed naturally, without any artificial assistance, and they are homogeneous in composition. They are usually composed of two or more elements which are chemically united. Diamond, being pure car bon only, is an exception.
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