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Diamond is a solid form of carbon with its atoms arranged in a crystal structure known as diamond cubic. It is metastable at standard temperature and pressure, converting to the chemically stable form graphite under those conditions but at a negligible rate. Diamond has the highest hardness and thermal conductivity of any natural material, properties that are useful in major industrial applications such as cutting and polishing tools. In addition to its industrial uses, diamond is much sought-after as a gemstone for use in jewelry. Most natural diamonds were formed between 1 billion and 3.5 billion years ago, at depths between 150 to 250 kilometres (93 to 155 mi) in the Earth's mantle. They were then carried to the surface more recently in volcanic eruptions and deposited in igneous rocks known as kimberlites and lamproites. This photograph shows the cathodoluminescence of a round-cut diamond, taken with the use of a scanning electron microscope. The image has a field of view of 3.45 millimetres (0.136 in).Photograph credit: Pavel Somov