Light sinks centimetres into marble, giving statues their lifelike glow
Sculptors have favoured white marble for the human body since classical times for an optical reason. Calcite lets light travel between about 13 and 38 millimetres into the stone before bouncing back out, producing a soft, waxy sheen that resembles skin far better than surface-reflecting rock.
Geologically, marble is limestone or dolomite transformed by heat and pressure. Metamorphism recrystallises the original carbonate grains into an interlocking mosaic, usually erasing the fossils and layering of the parent rock. Stonemasons use the word more loosely: in the building trade almost any polishable crystalline calcite rock counts, so the so-called Tennessee marble is really an Ordovician limestone that geologists name the Holston Formation.
Colour is a record of impurities. Pure white marble comes from exceptionally clean limestone or dolomite. The veins and swirls in coloured varieties started as clay, sand, silt, iron oxides or chert in the original sediment, which metamorphism mobilised and regrew. Green tones usually signal serpentine, formed from magnesium-rich rock containing silica. The name traces back to the Greek marmaros, shining stone, possibly linked to a verb meaning to gleam, and the same root gives English the adjective marmoreal.
The same chemistry that makes marble beautiful makes it vulnerable. Acids attack calcium carbonate, releasing carbon dioxide, so acid rain etches outdoor statues and gravestones and can coat them in a black crust of calcium sulfate, nitrates and soot. Cleaning with vinegar is a mistake for the same reason. Microbes join in too: a bacterium called Methylophaga murata was isolated from decaying marble in the Kremlin, and fungi have been found degrading stone at Milan Cathedral. Polishers exploit the reaction deliberately, treating floors with fluorosilicate and acid to bond a harder, glossier layer to the surface.
Marble is quarried in blocks by drilling, blasting, water jets or wedges. China, Italy, India and Spain together account for close to half of world output of marble and decorative stone, and Ashgabat in Turkmenistan entered the 2013 Guinness records for the densest concentration of white marble buildings anywhere.
Source: Marble
