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Thanks to their hardness, durability, and rarity, diamonds are symbols of eternal love and wealth. A new analysis of how these gems emerge from the depths where they were forged billions of years ago suggests that they should also evoke images of diamond-studded fountains of gas and rock.
Almost all diamonds come from volcanic deposits called kimberlites. Tens of thousands of such deposits, some more than a kilometer across, have been found protruding through Earth's surface. However, only one in every 200 of them contains gem-quality diamonds. Increasingly, scientists are looking at kimberlites' odd mix of geological characteristics to discern how diamond deposits get there in the first place.
With or without diamonds, kimberlites are enigmatic, says James W. Head III, a planetary geologist at Brown University in Providence, R.I. The carrot-shaped deposits, which can extend to depths of 2.5 kilometers, are clearly volcanic in origin, but areas around them aren't marked by large amounts of lava. Furthermore, kimberlites contain large numbers of glass spherules that typically form from airborne droplets of molten rock. Locked within kimberlites, however, the spherules apparently never make it above ground. Finally, as much as half the jumbled rocks in kimberlite deposits comes from Earth's mantle, the hot, viscous material that lies below depths of 35 km.