Diamonds are the only gemstone whose crystal is made from a single element: carbon. Deep within the Earth, at depths of at least 90 miles, carbon atoms are exposed to enough pressure and heat that they are squeezed into the tight and perfectly symmetrical alignment of diamond. If these conditions remain stable, the gems can happily exist in the Earth for extremely long periods of time - until special kinds of volcanic activity push them up to the surface. Some diamonds have inclusions that scientists can access to estimate the age of the stone. The oldest diamonds identified come from Canada and are 3.5 billion years old!