Christopher Arnheim, age 15, of Utica, N.Y., for his question:

WHAT IS LAPIS LAZULI?

Lapis lazuli is a beautiful azure blue mineral whose color makes it valuable as an ornament. The mineral is sometimes called lazurite.

Lapis lazuli consists chiefly of aluminum, sodium, silica and su . The mineral occurs in masses in fine grains and in crystals.

Most of these deposits are in beds of limestone.

Lapis lazuli sometimes is flecked with brilliant, shining spots. These spots are pyrite, which is an iron sulfide, and they help to identify genuine lapis lazuli.

Ancient Egyptians used lapis lazuli in their jewelry and so did the Romans.

The mineral was once the only source of ultramarine, a blue pigment used in artists' paints. This pigment is now made chiefly by chemical methods.

Ancient peoples believed this mineral had medical value. Some of them ground lapis lazuli stones to a powder, which was then mixed with milk. The mixture was used as a dressing for boils and ulcers.

The best specimens of lapis lazuli today come from Afghanistan.