|MadSci Network: Earth Sciences|
1 carat is, by definition, 200 miligrams or one-fifth of a gram. Since diamond is pure carbon, it takes one-fifth of a gram of pure carbon. Diamonds do not generally come from coal, but rather from graphite, which is also pure carbon. It takes one-fifth of a gram of graphite, therefore, to make one carat of diamond. Coal, if it could be buried and heated higly enough, would lose variable amounts of water, methane, and other non- carbon, so it would take a little more than one-fifth of a gram of coal, but I cannot give you a precise amount since the percentage of those other substances in coal can be quite variable. How long it takes to grow a diamond is a very difficult one to answer. I'm not sure that anyone really knows how fast natural diamonds grow. We do know that most are very old, but they could have formed very quickly and then just remained as they were for a long time until they finally found their way to the surface. The rate of mineral growth is determined by how quickly atoms are able to wiggle and squirm through the crystal structure of neighboring atoms and how far they have to travel. How fast they wiggle and squiggle (technically referred to as diffusion) depends directly on the temperature. Diamonds grow where it is hot, so diffusion will be fast, and they could likely grow in anything from a few years to a few thousand years. The rates of changes are a very important and interesting question in geology and one that a lot of geologists are trying to figure out - good question! Dave Smith
Try the links in the MadSci Library for more information on Earth Sciences.