Diamonds, those precious, sparkling jewels, are known as the hardest materials on Earth. They are a high-pressure form of carbon and found deep in the ground. While diamonds are commonly thought of as hard and stable, carbon from about 100 miles beneath the African plate is being brought to shallower levels where diamond will become unstable. Molten rock (magma) brings the excess carbon towards the surface, and earthquakes open cracks that allow the carbon to be released into the air as carbon dioxide.
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Source: ScienceDaily