Joe Urlaub, age 12, of San Bernardino, Calif., for his question:

How can you test whether a pearl is real?

The moon glow surface of a cultured pearl is just as real as that of a natural pearl. Both are made by the pearl oyster in the same way from the same materials. The same materials also line the inside walls of his shell. He builds the pearl by gleaming layer, to coat perhaps a gritty grain `of sand that has invaded his home.

There is no way to tell by looking which pearl is natural and which is artificial. The difference is inside, and we need an X ray machine to reveal it. The artificial pearl is built around a bead placed inside the oyster by the hand of man. The central core is larger, and its coat of pearly material is thinner than a natural pearl.