Diamonds almost always have defects and one of the main defects is carbon spots. Their influence upon diamond value has to be taken into account along with the influence of color. A stone may have great color, yet may yield some carbon imperfections making it far less valuable than an off color stone.
A truly colorless diamond is very rare, so it may be stated that one perfect in crystallization is also seldom. A perfect crystal is rarely found in nature. The diamond cutter tries to remove as many of these imperfections as possible, without cutting the weight of the actual stone. Usually after the stones are cut most of them still have some imperfections and blemishes.
One of the most common types of imperfection is known in the trade as carbon or as carbon spots. Diamonds are one of the many types of carbon. Graphite is a second variety, and amorphous carbon such as charcoal, lampblack is a third.
There is no hard line between the various forms of carbon. Artificial attempts to make diamonds have yielded both genuine diamond and also very hard, black, opaque carbon in the same lot, and even in the same crystal, and apparently in the natural formation of diamonds some of the material is frequently left in a black and opaque, but very hard, condition. Masses of black carbon are sometimes found in Brazil, they are sold under the name of carbonado. People often use the black carbon to be used in mechanical purposes due to its toughness and great hardness. This black carbon is often used for rock drill bits. South African mines contain some specks of black opaque carbon which can constitute very undesirable defects in stones.
Often times a large stone can have a large defect in it, and it will have to be cut into several different smaller pieces. Diamond cutters are often times after less sizable stones to eliminate carbon spots. One of the more serious effects that a carbon spot may have on a diamond is it lying in a more conspicuous spot, the light may hit this spot and reflect the carbon spot. This may make the stone appear to have many carbon spots instead of one. The cutter wants to cut the diamond in a way that puts the carbon spot in a less conspicuous place. Often times if the stone is going to be placed in a setting they will bout the carbon spot on a side of the fitting, thus covering the fitting.
When a carbon spot can be seen by the unaided eye, it is a very bad defect, and greatly affects the value of the stone. If a defect is very slight it may not affect the value of the stone too drastically. Many of the whitest diamonds seem to be afflicted with carbons.
Many of the fine blue-white stones are affected by carbons, while diamonds of the poorer colors are less likely to have carbons. A defect that may resemble a patch of carbon may result in cracks of the diamond. Diamond dust and oil can slip into a crack and create a defect. This problem may be avoided by using a nearly dry polishing wheel while working around cracks.
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