Diamonds

　When most people think of diamonds, they think of beautiful jewelry. But the beauty of diamonds is only one reason they are so ( 36 ).
　Another reason is that diamonds are the hardest natural objects on earth. Every year, 10 tons of diamonds are dug out of the ground. But only about 20% of those diamonds are large enough and good enough to be used for jewelry. The other 80% are called "industrial diamonds" and are used to create thousands of products, from eyeglasses to computer chips. Because industrial diamonds are so hard, they are used in many kinds of tools. ( 37 ), dentist's drills and machines for cutting metal and stone use industrial diamonds. Without industrial diamonds, many kinds of factories could not ( 38 ).
　Scientists have found another reason why people should be interested in diamonds --- they can ( 39 ) us many things about the history of the earth. Most diamonds were formed billions of years ago deep inside the earth. About 100 miles underground, there is a part of the earth called the mantle. Inside the mantle, very high temperatures and pressure turned a material called carbon into diamonds. During this process,other chemicals from the mantle often got caught inside the diamonds. Later, huge volcanoes, acting like elevators, brought the diamonds up from the mantle. So now scientists ( 40 ) diamonds can learn a great deal about what the mantle was like billions years ago.