The surface of marble crumbles readily when exposed to a moist, acid atmosphere, but marble is durable in a dry atmosphere and when protected from rain. The purest form of marble is statuary marble, which is white with visible crystalline structure. The distinctive luster of statuary marble is caused by light penetrating a short distance into the stone and then being reflected from the surfaces of inner crystals. The most famous variety of statuary marble is the Pentelic marble of Greece, found in the quarries of Mount Pentelikon in Attica. Pentelic marble was used by the great sculptors of ancient Greece, including Phidias and Praxiteles, and it was used to make the Elgin marbles. Parian marble, also used by ancient Greek sculptors and architects, was quarried chiefly at Mount Parpessa on the Grecian island of Paros. Carrara marble—which occurs abundantly in the Apuan Alps of Italy and is quarried in the region of Carrara, Massa, and Serravezza—was used in Rome for architectural purposes in the time of Augustus, the first emperor, but the finer varieties of sculptural marble were discovered later. The greatest works of Michelangelo are made of this marble, and it is used extensively by present-day sculptors.