单项选择题
About 150 years ago the children of a poor farmer who lived in South Africa found a pretty pebble (鹅卵石) on the bank of a river. The pebble shone brightly in spots. The children took the pebble home and showed it to their mother. Then they tossed (扔) it aside. The next day a neighbour saw it and offered to buy it. The children and the mother told him that he could have it for nothing. Who ever heard of selling a pebble
The pebble turned out to be a large diamond. No one had known that there are diamonds in South Africa. Today a large part of all the diamonds in the world come from a region not far from the place where the children found their bright pebble.
Diamonds are crystals of carbon. Carbon is a very common material. Coal, for example, is made up mostly of carbon. But the carbon of coal is not in crystal form. Clear crystals of carbon are very rare.
The word "diamond" comes from a Greek word meaning "the unconquerable". The diamond got its name because of its hardness. No other material is so bard. It is used in tools for cutting and drilling into very hard substances. Some saws, for example, have tiny diamonds set in the teeth.
A. Rareness
B. hardness
C. crystal form
D. bright spots