Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism. It is composed primarily of the mineral calcite (CaCO3) and usually contains other minerals, such as clay minerals, micas, quartz, pyrite, iron oxides, and graphite. Under the conditions of metamorphism, the calcite in the limestone recrystallizes to form a rock that is a mass of interlocking calcite crystals. A related rock, dolomitic marble, is produced when dolostone is subjected to heat and pressure.Pure white marble is the result of metamorphism of a very pure (silicate-poor) limestone or dolomite protolith. 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 magnesium-rich limestone or dolostone with silica impurities. These various impurities have been mobilized and recrystallized by the intense pressure and heat of the metamorphism.According to the United States Geological Survey, U.S. domestic marble production in 2006 was 46,400 tons valued at about $18.1 million, compared to 72,300 tons valued at $18.9 million in 2005. Crushed marble production (for aggregate and industrial uses) in 2006 was 11.8 million tons valued at $116 million, of which 6.5 million tons was finely ground calcium carbonate and the rest was construction aggregate. For comparison, 2005 crushed marble production was 7.76 million tons valued at $58.7 million, of which 4.8 million tons was finely ground calcium carbonate and the rest was construction aggregate. U.S. dimension marble demand is about 1.3 million tons. The DSAN World Demand for (finished) Marble Index has shown a growth of 12% annually for the 2000�2006 period, compared to 10.5% annually for the 2000�2005 period. The largest dimension marble application is tile.

Facts About Marble:

1. Marble is a metamorphic stone. It starts life as limestone or dolomite that�s buried in the Earth. Over hundreds of years, immense pressure and high heat compacts and transforms the original limestone. The result is large blocks of marble that are harder, denser and more colourful than the original stone.
2. The distinctive veining patterns are one of the most recognisable elements of marble stone. These patterns are created by small minerals that fill cracks and fissures in the original limestone. As the stone is subjected to pressure, the minerals crystalise, creating those reflective, glistening veins throughout.
3. Marble�s density and durability are key reasons the Taj Mahal and the Pantheon still stand today. On the Mohs rating scale, marble ranks a 2.5 to 5, meaning it�s about average in terms of hardness, but when properly installed, it can last generations. (Comparatively, granite ranks about a 7 to 8.)
4. I got this information off of Marble Matters which you can visit here.