Diamond Formation
Learning about diamonds’ formation process is essential to understanding its rarity and value. With this new knowledge you’ll appreciate even more the extraordinary nature of the diamonds.
Extremes of temperature and pressure can cause dramatic changes in any substance. Temperature and pressure are even more powerful when they work together. The combination is what causes diamond’s atoms to link together in the special way they do. This unique bonding of its atoms is what makes diamond the hardest substance on earth. Diamond is composed of only carbon, and the way its carbon atoms fit together makes it way harder than any other material. Graphite is also made of carbon, but in diamond the atoms are arranged in tight three-dimensional patterns, bonded more strongly—in all directions—than graphite’s atoms.
The best temperature and pressure conditions for diamond formation exist under large, geologically stable parts of the crust called cratons. Cratons are the oldest sections of the landmasses. They make up the interior portions of the continents, where they’ve existed without change for millions of years. The basic requirements for diamond formation are precise temperature and pressure conditions in the right combination, and an environment with sufficient carbon. By studying the inclusions in diamond crystals as well as the rocks that accompany diamonds to the surface, scientists found that diamonds form in two different types of rocks. Peridotite (pe-RID-o-tite) is an igneous rock, while eclogite (EC-lo-gite) is a metamorphic rock. Igneous rocks start out in a molten or partially molten state and become solid when they cool. Metamorphic rocks are pre-existing rocks that have been altered by heat and pressure. In both peridotite and eclogite, if temperature and other factors like pressure and chemistry are favorable, the carbon atoms released by the melting rock bond to build diamond crystals.
How Diamonds Reach The Surface
After formation, if conditions remain constant, diamonds might remain underground for hundreds of millions of years before they’re carried to the surface. But the delivery itself is rapid. It has to be: Under the inner earth’s extreme heat conditions, any diamonds that spend too long under decreased pressure won’t survive.
Diamond deposits are found in two types of rock: kimberlite and lamproite. Kimberlites and lamproites have been pushed up to the earth’s surface for billions of years. Sometimes, they carried diamonds with them. The process, whether the materials contain diamonds or not, is called emplacement. It’s important to remember that gem-quality diamonds don’t form in kimberlites and lamproites. These rocks merely carry already-formed diamonds to the surface. Heat trapped deep within the earth keeps portions of the mantle in a melted state, the molten rock material is called magma. If the mixture of chemical elements is right, the magma will form kimberlite or lamproite. If, as it rises, it meets an already-existing deep fracture in the crust, it will continue to move upward. If the rising kimberlite or lamproite passes through a diamond-bearing area of a craton, it will pick up and carry already-formed diamonds. The combination of speed and expanding gasses is powerful enough to force the rising rock mixture to explode through the surface. As kimberlite blasts through the crust, it creates a deep carrot-shaped formation called a pipe. The kimberlite—or lamproite—doesn’t flow out of the pipe like lava from a volcano. It solidifies while it’s still very hot, so by the time it reaches the surface it has changed from liquid to solid. The explosion is a mixture of solid rock, ash, and gasses. After the material explodes through the overlying rock, most of it falls back into its pipe, along with its diamond cargo.
Kimberlites are widespread—in the 1990s, there were about 6,000 of them known worldwide. But fewer than 1,000 of them contained any diamonds. Of those, only about 50 had enough diamonds to be economic to mine, and only about 20 are still being mined today. With statistics like that, it’s no wonder diamonds are considered rare and precious!
Source: Gemological Institute of America
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