This is a photomicrograph (a couple of mm across) of a garnet with inclusions of silica. Most silica at the earth's surface is in the form of quartz. But under high pressures (equivalent to depths in the earth in excess of 80 km), the stable form of silica is the mineral coesite. The garnet crystal has acted as a protective pressure vessel so that pieces of coesite have been preserved to the earth's surface. The attempted change to quartz has tried to expand the inclusion - causing radial cracks in the garnet. This classic image (provided by Christian Chopin) is from the Dora Miara internal basement massif. So this fragment of continental crust was once over 80 km down in the earth.

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