Donna Smith, age 12, of Pocatello, Ida., for her question:

JUST WHAT IS ONYX?

Onyx is a term used loosely to apply to a banded marble and also to agate, a fine Brined variety of quartz. Onyx is strong and hard and takes a high polish. It is widely used in the carving of cameos and intaglios.

Ordinary onyx of quartz or agate is dyed black and white, green and white or red and white. Sardonyx is brown and white onyx.

Today, jewelers refer to dyed, single color agate as onyx. When they speak simply of onyx, they mean the black stone. Green onyx is the same stone dyed green.

Onyx marble or Mexican onyx is a variety of calcite marble which is found on the walls of caves. Mexican onyx shows a banding like that of agate, but it is much coarser. The colors of Mexican onyx range from white to green, red and brown.

Much of the Mexican onyx is cut into gemstones, colored with an aniline dye, then set in inexpensive native silver jewelry. The stones are brittle and not durable.

Mexican onyx is also used as a decorative stone.