Diamonds are among the most sought-after, and chemically simple, objects on the planet. They are used in many applications, from electronic devices to the edges of diamond blades. They can be naturally occurring or man-made, and they come in various sizes, shapes and colors. Natural diamonds are formed from the element carbon in a slow and ever-occurring geological process.
Pure diamonds are pure carbon, although most diamonds contain some impurities. The graphite in lead pencils is also formed from carbon, and this versatile element, which is essential for living organisms on Earth, forms innumerable bonds with other elements. The formation of diamonds from raw carbon requires a great amount of heat and pressure. Suitable environments occur naturally deep beneath the earth in thick layers of crust known as cratons. In addition, very tiny diamonds have been found at meteor impact sites, where intense impact pressure and heat cause ideal conditions for a very short amount of time.
Nitrogen can be present in diamonds in trace amounts as an impurity because nitrogen atoms are capable of replacing carbon atoms in the crystal lattice that gives diamonds their structure. The presence of nitrogen causes the diamonds to absorb blue light, making the stones appear yellow. Nitrogen content is lower in diamonds that have been formed under higher pressure and for greater periods of time.
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When boron is present in trace amounts, it's responsible for the blue-gray coloration of some diamonds. Diamonds with boron in them are known as Type 2-A diamonds. The presence of boron also enables these diamonds to behave as semiconductors -- diamonds are ordinarily electrical insulators. Boron nitride, a compound used in bulletproof vests, is nearly as hard as diamond, and is in fact more chemically stable.
Type 1-A diamonds are high in hydrogen content. These diamonds also appear blue, like 2-B diamonds, but are electrical insulators rather than semiconductors. It is not certain whether the hydrogen is responsible for the color or not. The Argyle diamond mine, located in northern Australia, produces a large amount of type 1-A diamonds.