The Travertine reference article from the English Wikipedia on 24-Jul-2004
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Travertine

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Travertine Hot Springs

Travertine is a white concretionary form of calcium carbonate that is usually hard and semicrystalline. It is deposited from the water of mineral springs (especially hot springs) or streams holding lime in solution. Extensive deposits exist at Tivoli, Italy, near Rome. The largest building in the world constructed largely of travertine is the Colosseum in Rome, Italy