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There are two types of rock that typically host diamonds. Kimberlites and Lamproites are igneous rocks that originate deep within the mantle (usually 150-450 km deep). Diamonds form within the mantle when carbon is subjected to extremely high pressures, and then they arrive at the surface when magma from these depths carries the diamonds up to the surface. The magma then cools into either Kimberlite or Lamproite with diamonds trapped inside. There is an additional rock type that can contain diamonds: Eclogite. Unfortunately, the diamonds in Eclogite are generally too small to be of economic worth so Eclogite is famous for other reasons. Eclogite is one of the densest rocks known to man – its density is hypothesized to be one of the drivers for plate tectonics. Eclogite forms as basaltic rock is subducted, and exposed to extremely high pressure within the mantle. For an Eclogite to form, the basalt needs to be subducted to a depth of at least 45 km.