From Halopedia, the Halo wiki
Marble is a non-foliated metamorphic rock resulting from the metamorphism of limestone, composed mostly of calcite (a crystalline form of calcium carbonate, CaCO3). Marble is known for its hardness, durability, and glossy properties. It is extensively used for sculpture, as a building material, and in many other applications.
Humans as of the 26th century occasionally use marble as a building material; for example, the Côte d'Azur Museum of Natural History, built to resemble an ancient Greek temple, was constructed mainly of marble.
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