Cycladic figurine from Greece, ca. 2000 BC
Marble is a metamorphic rock made out of limestone. When limestone is subjected to tremendous pressure for a long time (like if limestone is buried under a lot of other rock or an ocean) it gets squashed into marble. Marble is more beautiful than limestone and tougher, and so people like to use it for buildings.
Multicolored marble on the floor of the
Roman Senate house
But marble is also rarer, and more expensive. A lot of marble is white, but marble can come in all different colors. In ancient Greece and Rome, people used marble (especially white marble) to make statues, and they used colored marble in patterns to make hard floors that would last a long time. Different colors of marble came from different parts of the Roman empire - the purple came from Egypt, for instance - and so this was also a way of showing off, of pointing out how powerful Rome was, that the Senate could bring stone from all these far away places that were ruled by Rome.
Carving a marble statue
Sometimes people also used marble in thin sheets on the walls of fancy buildings like churches or palaces, to make a brick wall look fancier. When marble was too expensive, people used plaster frescoes on their walls that were painted to look like marble.
To find out more about marble, check out these books from Amazon.com or from your library:
DK Eyewitness Guides: Building, by Philip Wilkinson (2000).
Geology: A Golden Guide from St. Martin's Press, by Frank Rhodes (2001).
Ancient marble quarrying and trade (1986). A collection of papers by specialists, for specialists, about marble in ancient Greece and Rome.