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时间：2018-12-01 作者：英语课 分类：自考英语（一）78讲
Scientists know that the combination of extreme heat and pressure changes carbon into diamonds. Such heat and pressure exist only in the hot, liquid mass of molten rock deep inside the earth. It is thought that millions of years ago this liquid mass pushed upward through cracks in the earths crust. As the liquid cooled, the carbon changed into diamond crystals.
There are only four areas where very many diamonds have been found.
The first known(famous) area was in India, where diamonds were found thousands of years ago. In the 1600s, travelers from Europe brought back these beautiful stones from India. Diamonds became very (be)popular with the kings and queens of Europe.
In the 1720s, diamonds were discovered in Brazil. This discovery came at a good time, too. Indias supply of diamonds was finally running out after 2,500 years of mining the stones.
In the 1800s, two other important areas were found in Russia and South Africa. Today, most diamonds used in industry come from Russia. Most diamonds used as gems 1 come from South Africa. only twenty-five percent of all diamonds mined are good enough for cutting into gems.
Most of the diamonds in India were found in stream beds. people would pick up handfuls of gravel 2 from the bottom of the streams and sort out the diamonds. These diamonds were probably carried from where they were formed to India by great sheets of moving ice that covered parts of the earth 20,000 years ago.
Most diamonds today are not found in stream beds, however. They are mined from rock formations deep inside the earth called pipes. Scientists believe that these are parts of volcanoes that were formed when molten rock pushed upward through the earths crust. The hard rock in which diamonds are found is called blue ground, because it is somewhat blue. The blue ground is blasted into large pieces of rock which are carried to the surface by elevator. Then the rocks are carefully crushed so that the diamonds are not destroyed. Next, the crushed material is taken over to washing tables. Here, it flows over boards thickly coated with grease. Since diamonds stick to grease, they are left behind by the rocks and mud which flow down the tables.
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I will pick out 10 of you to attend the contest.