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  3. Diamonds are just carbon in its most concentrated form, the element that makes up 18 percent of the weight of your body. There i

Diamonds are just carbon in its most concentrated form, the element that makes up 18 percent of the weight of your body. There i

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问题     Diamonds are just carbon in its most concentrated form, the element that makes up 18 percent of the weight of your body. There is no other gemstone as cherished as the diamond, but in truth, diamonds are no rarer than many other precious gems. They continue to demand higher market prices because the majority of the diamond market is controlled by a single entity.
                            The Origin of Diamonds
    Carbon is one of the most common elements in the world, and is one of the four essentials for the existence of life. Humans are more than 18-percent carbon. When occurring in nature, carbon exists in three basic forms, of which diamond is an extremely hard, clear crystal.
    Diamonds form about 161 km below the Earth’s surface, in the molten rock of the Earth’s mantle, which provides the right amounts of pressure and heat to transform carbon into a diamond. In order for a diamond to be created, carbon must be placed under at least 435,113 pounds per square inch (psi) of pressure at a temperature of at least 400 Celsius.  If conditions drop below either of these two points, graphite will be created. At depths of 150 km or more, pressure builds to about 725,189 psi and heat can exceed 1,200 C.
                            Kimberlite Pipes
    Most diamonds that we see today were formed millions (if not billions) of years ago. Powerful magma eruptions brought the diamonds to the surface, creating kimberlite pipes. Kimberlite is named after Kimberly, South Africa, where these pipes were first found. Most of these eruptions occurred between 1,100 million and 20 million years ago.
    Kimberlite pipes are created as magma (岩浆) flows through deep fractures in the Earth. The magma inside the kimberlite pipes acts like an elevator, pushing the diamonds and other rocks and minerals through the mantle and crust in just a few hours. These eruptions were short, but many times more powerful than volcanic eruptions that happen today.
    The magma eventually cooled inside these kimberlite pipes, leaving behind conical veins of kimberlite rock that contain diamonds. Kimberlite is a bluish rock that diamond miners look for when seeking out new diamond deposits.  The surface area of diamond-bearing kimberlite pipes ranges from 2 to 146 hectares. Diamonds may also be found in river beds, which are called alluvial diamond sites. These are diamonds that originate in kimberlite pipes, but get moved by geological activity. Glaciers and water can also move diamonds thousands of miles from their original location.
                            Cutting Diamonds
    There are special techniques that are used to cut and shape a diamond before it gets to the jewelry store. Diamond cutters first try to remove any impurities or irregularities in the diamond. Sometimes, diamonds have to be sawed with the use of a phosphor-bronze blade or laser. Then the diamond is rubbed by another diamond to create facets-tiny flat spots on the surface. Finally, the diamond is pressed against a rotating polishing wheel to give the diamond its finished look.
                            Rating Diamonds
    Diamonds are judged on several factors that determine their beauty.  Most diamonds never reach the consumer market because they are too flawed. Often, these diamonds are used for industrial purposes--as an abrasive, for drill bits or for cutting diamonds and other gems. If you’ve ever purchased a diamond, you’ve heard of the "4 Cs:"
    1. Cut--This refers to how the diamond has been cut and its geometric proportions. When a diamond is cut, facets are created and the diamond’s finished shape is determined.
    2. Clarity--This is the measurement of a diamond’s flaws, or inclusions that are seen in the diamond.
    3. Carat--This is the weight of a diamond. One carat is equal to about 200 milligrams.
    4. Color--In referring to transparent diamonds, the color scale runs from D to Z, beginning with Icy White-- the color of the most expensive diamonds--and ending with a light yellow.
    Other unique qualities of the diamond include its transparency, luster and dispersion of light. A diamond that is created from 100-percent carbon will be completely transparent. Diamonds often contain other elements that can affect the color. Although we often think of diamonds as being clear, there are also blue, red, black, pale green, pink and violet diamonds. These colored diamonds are the truly rare ones.
                            Marketing Diamonds
    Prior to the 1930s, diamond rings were rarely given as engagement rings. Opals, rubies, sapphires and turquoise were deemed much more exotic gems to give as tokens of one’s love.  This idea of connecting diamonds to romance was captured in a brilliant ad campaign begun in the 1940s, causing demand for diamonds to increase.  Surely you’ve heard the De Beers advertisement telling you that "A Diamond Is Forever." This ad campaign, which was created by the N.W. Ayer advertising agency in 1947, changed the diamond market. In 2000, Advertising Age magazine named the ad campaign the slogan of the 20th century.
    Later ads by De Beers told consumers to hold onto their family’s diamond jewelry and to cherish them as a family possession--and it worked. This eliminated the aftermarket for diamonds, which further enabled De Beers to control the market. Without people selling their diamonds back to jewelers or to other people, the demand for new diamonds increased.
    There are fewer than 200 people or companies authorized to buy rough diamonds from De Beers. These people are called sightholders, and they purchase the diamonds through the Central Selling Organization (CSO), a subsidiary of De Beers that markets about 70 percent to 80 percent of the world’s diamonds. De Beers sells a parcel of rough diamonds to a sightholder, who in turn sends the diamonds to cutting facilities and then to distributors.  There are rough diamonds sold outside the CSO.  However, the cost of these diamonds is still largely influenced by the prices set by the CSO.
    Diamonds are the most coveted of all precious gems, as is witnessed by the extremely high demand for them. While this has not always been the case, diamonds are nonetheless exquisite gems that go through a long, tedious refining process from the time they are pulled from the ground to when you see them in the jewelry store. And, while some of the mystique of diamonds may be gone--they’re just carbon, after all--the diamond will likely continue to be a highly coveted jewel, because, well, "A Diamond Is Forever."
Carbon can transform into diamonds with appropriate ______.
选项
答案pressure and heat
解析 在“钻石的来源”这一部分中，作者说明了在适当的压力和温度下，碳可以转化成钻石。
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