Precious stones

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A gemstone is a mineral, rock, such in the case of lapis lazuli, or petrified material that when cut or faceted and polished is collectible or can be used in jewellery. Others are organic, such as amber, which is fossilized tree resin and jet, a form of coal. Some gemstones which may be generally considered precious or beautiful are too soft or too fragile to be used in jewelry, for example, single-crystal rhodochrosite, but are exhibited in museums and are sought by collectors.
Minerals are natural compounds formed through geological processes. The term "mineral" encompasses not only the material's chemical composition but also the mineral's structure. Minerals range in composition from pure elements and simple salts to very complex silicates with thousands of known forms (organic compounds are usually excluded). The study of minerals is called mineralogy.

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To be classified as a "true" mineral, a substance must be a solid and have a crystal structure. It must also be an inorganic, naturally-occurring, homogeneous substance with a defined chemical composition. The chemical composition may vary between end members of a mineral system. For example the plagioclase feldspars comprise a continuous series from sodium-rich albite (NaAlSi3O8) to calcium-rich anorthite (CaAl2Si2O8) with four recognized intermediate compositions between. Mineral-like substances that don't strictly meet the definition are sometimes classified as mineraloids. Other natural-occurring substances are Nonminerals. Industrial minerals is a commercial term and refers to commercially valuable mined materials (see also Minerals and Rocks section below).
A crystal structure is the orderly geometric spatial arrangement of atoms in the internal structure of a mineral. There are 14 basic crystal lattice arrangements of atoms in three dimensions in the six crystal systems, and all crystal structures currently recognized fit in one of these 14 arrangements. This crystal structure is based on regular internal atomic or ionic arrangement that is often visible as the mineral form. Even when the mineral grains are too small to see or are irregularly shaped the crystal structure can be determined by X-ray analysis and/or optical microscopy.

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Chemistry and crystal structure define together a mineral. In fact, two or more minerals may have the same chemical composition, but differ in crystal structure (these are known as polymorphs). For example, pyrite and marcasite are both iron sulfide. Similarly, some minerals have different chemical compositions, but the same crystal structure: for example, halite (made from sodium and chlorine), galena (made from lead and sulfur) and periclase (made from magnesium and oxygen) all share the same cubic crystal structure.
Crystal structure greatly influences a mineral's physical properties. For example, though diamond and graphite have the same composition (both are pure carbon), graphite is very soft, while diamond is the hardest of all known minerals.
There are currently just over 4,000 known minerals, according to the International Mineralogical Association, which is responsible for the approval of and naming of new mineral species found in nature.
A mineral is a naturally occurring, inorganic solid with a definite chemical composition and a crystalline structure. A rock is an aggregate of one or more minerals. (A rock may also include organic remains.) The specific minerals in a rock can vary a lot. Some minerals, like quartz, mica or feldspar are common, while others have been found in only one or two locations worldwide. Over half of the mineral species known are so rare that they have only been found in a handful of samples, and many are known from only one or two small grains.
Commercially valuable minerals and rocks are referred to as industrial minerals.

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