John Sowell, age 11, of Huntsville, Ala., for his question:

WHAT IS A CARAT?

A carat is a jeweler's measure for weighing pearls and precious stones. The word comes from the Arabic carat, meaning bean or seed. Carob tree seeds, in ancient times, were used as weights for precious stones, which were described as being so many carates.

Metric carat weight is 200 milligrams, or 0.2 gram.

Carat is also used to express the amount of gold in an alloy, a carat being 1/24 of the total weight of the alloy. This means if 18/24 of an alloy is gold, it is 18 carats fine. Pure gold is called 24 carats fine.