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Characteristics and classification
Synthetic and artificial gemstones
Gemstone list
Ruby
Sapphire
Emerald
Amethyst
Characteristics and classification
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.
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.
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