They glitter on fingers and in ears. They decorate watches, bracelets and necklaces. But did you know diamonds are also used to cut glass, to polish metal and to drill through rock?

Doctors use diamond-edged knives during surgery. Jewelers cut other gem stones with diamonds. They are an important material in all kinds of industry -- especially car and airplane manufacturing. They're also used in electronics.

Diamonds have a special quality that makes them so useful: They're the hardest material found in the natural world. Diamonds are minerals. They're made of carbon, the same material that forms coal. Carbon is an element -- a material that can't be broken down into other, simpler substances. There are only 92 elements, and carbon is one of the most common.

Within elements, atoms arrange themselves in regular patterns that build up into solid forms called crystals. Diamonds form from carbon within igneous rock -- the hot, molten rock that originates deep within the earth. Under very high pressure, at very high temperatures and over very long periods of time, some of the carbon in the rock crystallizes into diamonds.

Most diamonds form about 100 miles beneath the surface of the Earth. It's hot down there: about 3,200 degrees Fahrenheit. Pressure reaches 2 million pounds per square inch. Under these extreme conditions, rocks melt and change. And all the ingredients for "cooking" diamonds are in place.

Diamonds are found below the surface in rock formations called pipes. These formations are the cooled remains of igneous rock that moved upward toward the earth's surface. The stones also turn up in the gravel in stream beds and in sand dunes along the coast. These deposits happened when rock containing diamonds weathered, or wore away. Then the diamonds were eroded, or transported by water or some other force, to another place.

Diamonds have even been found in meteorites from space! Scientists speculate that the heat and pressure of the meteorites' impact on Earth's surface created these tiny diamonds.

Experts think that the first diamonds were discovered in India about 3,000 years ago. One of the world's most famous diamonds, the Koh-i-Noor, is from there. Koh-i-Noor means "mountain of light." This diamond has been around since the 1300s and was once an important part of a Maharajah's throne in India. In 1849, it was bought by Britain and became part of the Queen of England's collection, the Crown Jewels, where it remains today.

People have always been fascinated by diamonds -- and superstitious about them. Long ago, soldiers thought wearing diamonds would protect them in battle. It was also thought that diamonds could prevent mental illness or cure people who had been poisoned.

Since the 1950s, scientists have been working on ways to make synthetic, or artificial, diamonds that are hard enough to use in industry. They've had a lot of success, although many of the methods are still too expensive to be useful. An exciting recent development has been the invention of artificial diamond film, which scientists can "grow" in a lab. These films can coat metal surfaces, which makes them extremely tough and durable.

These days, diamonds are big business. According to the Diamond Information Center in New York City, the four top mining nations are Australia, Botswana, the Soviet Union and Zaire. More than three quarters of the diamonds mined come from these four places.

If you saw a diamond that came straight out of the ground, you probably wouldn't be too impressed. Uncut diamonds look shiny, but they don't glisten. The stones don't get their characteristic sparkle until they have been cut. A cut diamond has many flat surfaces called facets. The facets reflect the light that hits the stone. The lower facets bounce light back out of the diamond, making it shimmer and twinkle. The shape and angle of the facets are what bring out a diamond's ability to gleam and flash.

Diamonds can also break white light up into colors -- a quality jewelers call "fire." Does your mom have an engagement ring? Ask her to move her hand around in the light; you'll see tiny rainbows reflected on surfaces nearby.

In the District, you can visit one of the world's most famous diamonds. The Hope diamond -- on display in the gem hall of the Smithsonian's Museum of Natural History on the Mall -- is the largest blue diamond in the world.

Tips for Parents

Kids can grow crystals using common household materials such as salt or sugar. For example, make a solution using 1/2 cup of hot water and salt. Add salt to the hot water and stir until no more will dissolve. Then suspend a pipe cleaner or a string from a pencil placed across the cup. Leave it undisturbed. After a few hours, crystals will already have begun to form. In a few days, most of the salt will have crystallized around the pipe cleaner or string. If you do this with a sugar solution, you can eat the product as well as admire it! For other fun science activities using ordinary household materials, see "Science Sensations" by Diane Willow and Emily Curran (Addison-Wesley; $8.95). This new paperback contains activities developed at the Children's Museum in Boston.

Catherine O'Neill is a freelance children's writer.