Every chemical substance which is part of the earth's crust is a mineral. Table salt, which may be mined or purified from salt water, is a mineral. Even petroleum as it comes from oil wells is classed as a mineral, as is the soft yellow material sulfur, mined in Louisiana and Texas. As can be seen from these examples, minerals may be either simple or complex substances. If a mineral is found to have only one kind of atom, it is a simple mineral
element. But if a mineral is made up of two or more elements chemically united into one new substance, it is a mineral compound. To go one step further, rocks, to the geologist, are natural samples of one or more minerals, formed in the earth in a particular way. For example, calcite is crystalline calcium carbonate. But when calcium carbonate is deposited under oceans as a commonly dull, gray sediment, it is called limestone.
Minerals often form in the earth as regular shapes called crystals. Examples of common crystalline minerals are fluorite, galena, pyrites, calcite, and quartz. Many of the crystalline minerals are used for jewelry because they are scarce or fashionable, brilliant in luster, clear and transparent, beautiful in color, or very hard and long lasting.
The diamond is an example of a valuable and beautiful mineral. It is the hardest known mineral. Thus it can be cut only with other diamonds. Ancient stonecutters found that by cutting and polishing a diamond in a certain way, ligth could be made to bounce from several sides inside the diamond before it reflected back to the eye,
giving the typical sparkle. Other precious minerals are sapphire,
emerald, and ruby.
