For example, there are several different varieties of silica. Silica is silicon dioxide. The most common is quartz, but there are other varieties that are formed at high pressures, and, in fact, some of these, the mineral called coesite, for example, has been identified in impact craters where meteorites smashed into the Earth, and this quartz in the rocks was reformed into a different crystal structure to form the new mineral called "coesite". Even ice, which we're all familiar with, has many different structures. The most common, of course, is the one that you find in your freezer, but under different conditions of temperature and pressure, ice can actually form several different crystal structures.

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