Matt Yost, age 9, of Lancaster, Pa., for his question:

WHAT ARE DIAMONDS MADE OF?

Diamonds are graded by color, purity, weight and cut. The weight is measured by the carat, with one carat weighing 200 milligrams. Purity means there aren't many small bubbles or cracks. Ideal color is a faintly yellowish tint, although some fine diamonds are colorless or have a tinge of blue. A well cut stone will best display its brilliance.

Hardest naturally occurring substance known to man is the diamond. And it is also one of the most valuable.

Diamonds are used in industry for boring, grinding and cutting because the stone is so strong. Eighty percent of all diamonds end up in industry, which means that only 20 percent find their way to jewelry.

Diamonds are crystals formed almost completely of carbon. Most diamonds have eight sides which form a double pyramid, although some have twelve sides and others six. Most scientists agree that diamonds were formed millions of years ago when carbon was subjected to great pressure and heat.

Africa supplies about 80 percent of the world's diamonds. Sixteen percent of the world's supply comes from deposits in Siberia.

First diamonds were found in the sand and gravel of stream beds. Later, gems were discovered in rock formations deep in the earth. These formations, called pipes, are thought to be the throats of long extinct volcanoes. Diamonds are found in rock called blue ground. Many tons of blue ground must be taken from the earth and then crushed and sorted to obtain one small diamond. It takes about three tons of blue ground to obtain one carat.

Today a number of companies manufacture man made industrial diamonds. Most are no larger than a grain of sand.

Imitation diamonds are also being made and sold today. Some are made from minerals, such as spinel. Others, such as glass strontium titanate and yttrium aluminate, do not exist in nature and are completely man made. Imitation diamonds do not have the same hardness as natural diamonds, so they eventually become scratched and show other signs of wear.

Diamond cutters learned how to polish and shape a stone with an iron wheel coated with diamond dust in the 1400s. Cutting was started during the 1600s.

Single diamonds have been found in widely separated parts of the United States. The only diamond mine in America is one located near Murfreesboro, Ark.