Vincent D. Manti, Age 11, of Albany, N.Y., for his question:

What is marble?

For thousands of years marble has been rated as the most glamorous of all building stones. The ancients used it to make statues and columns, floors and walls. Modern decorators imitate the graceful designs in marble to make vinyl and other plastic floor coverings. In nature, those designs were patterned from impurities trapped in translucent stone.

In the building world, marble is a rock made of the mineral calcium carbonate which can be polished. The fact that it can be polished to a shiny surface disqualifies the limestones from being marble, though they also are made of calcium carbonate. In the world Of geology, marble is a metamorphic rock, usually made from the fossilized shells of ancient crustaceans. Limestone is a sedimentary rock made from the shells of ancient crustaceans.

There are countless deposits of marble in the earth's crust, and no two deposits are exactly alike. Pure marble is translucent, like foggy glass, and tinted with milky white. Some marbles contain impurities which tint the milky stone with soft Yellow or rosy pink. In some deposits, green foliage  or purple violets seem to be frozen within the glassy stone. Many milky deposits are streaked and mottled with smoky black smudges of graphite.

These impurities which add beauty to the marble Were gathered as the marble was formed in the earth's crust. Some marble may have formed from calcium carbonate deposited by water. But most of it was formed from the shells of ancient crustaceans. These miniature shellfish left piles of their shells on the floors of the seas. The sea beds lifted and dried, and the layers of shells mixed with oozy silt became beds of Limestone.

In some cases, the limestone suffered further changes. It was buried under new layers of minerals and changed or metamorphosed. Under heat and pressure, its particles were re crystallized. And the coarse limestone became fine marble.

The Classic Age of Greece is remembered for its splendid statues. But its artists were able to reach such a high peak because, in the geological past, the Mediterranean region was under a shallow sea. As the earth's surface changed, limestone formed and many deposits of limestone were changed to marble. The Greeks found plenty of fine marble close at hand from which to practice the art of sculpture.