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
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 (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
Commercially valuable minerals and rocks are referred to as
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