 Marble is metamorphosed limestone,
composed of fairly pure calcite (a crystalline form of calcium
carbonate, CaCO3). It is extensively used for sculpture, as an
building material, and in many other applications. This metamorphic
process causes a complete recrystallization of the original rock
into an interlocking mosaic of calcite and/or
dolomite crystals. The temperatures and pressures necessary to
form marble usually destroy any fossils and sedimentary textures
present in the original rock.
Pure white marble is the result of metamorphism of very pure
limestones. The characteristic swirls and veins of many colored
marble varieties are usually due to various mineral impurities
such as clay, silt, sand, iron oxides, or chert which were originally
present as grains or layers in the limestone. Green coloration
is often due to serpentine resulting from originally high magnesium
limestone or dolostone with silica impurities. These various
impurities have been mobilized and recrystallized by the intense
pressure and heat of the metamorphism.
Some historically important kinds of marble, named after the locations
of their quarries, include:
* Carrara (Italy)
* Pentelicus (Greece)
* Proconnesus (Turkey)
 White
marbles, like Carrara, have been prized for sculpture since
classical times. This preference has to do with the softness
and relative isotropy and homogeneity, and a relative resistance
to shattering. Also, the low index of refraction of calcite
allows light to penetrate several millimeters into the stone
before
being scattered out, resulting in the characteristic "waxy" look
which gives "life" to marble sculptures of the
human body.
In
the construction trade, the term "marble" is used
for any massive, crystalline calcitic rock (and some non-calcitic
rocks) useful as building stone. For example, Tennessee Marble is really a massive, highly fossiliferous gray to pink to
maroon Ordovician dolostone, known as the Holston Formation
by geologists.
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