What is Marble ?

Taj Mahal, world-famous monument made of marble. Marble is a non foliated metamorphic rock resulting from the metamorphism of limestone, composed mostly of calcite (a crystalline form of calcium carbonate, CaCO3). It is extensively used for sculpture, as a building material, and in many other applications.

Etymology
Venus de Milo, front.The word "marble" derives from the Ancient Greek μάρμαρον (mármaron) or μάρμαρος (mármaros), "crystalline rock", "shining stone"[1], perhaps from the verb μαρμαίρω (marmaírō), "to flash, sparkle, gleam". This stem is also the basis for the English word marmoreal, meaning "marble-like."

Origins
Marble is a metamorphic rock resulting from regional or, rarely, contact metamorphism of sedimentary carbonate rocks (either limestone or dolomite rock) or metamorphism of older marble. This metamorphic process causes a complete recrystallization of the original rock into an interlocking mosaic of calcite, aragonite 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.

More: http://en.wikipedia.org/wiki/Marble