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Volcanic Rocks
The majority of the crystals on Earth are found in areas where the molten interior of the earth has been able to cool. During this cooling process, magma or lava can crystallize. The mix of minerals in the molten rock, as well as the temperature and pressure it is exposed to, determine what type of crystals are formed. These are typically found near volcanoes, hot spots or tectonic boundaries.
Sedimentary Rocks
The crystals formed in sedimentary rocks are entirely different in their origins than in igneous rocks. Dissolution is a key component of crystal formation, particularly in karst, or carbonate cave, environments. Dolomite, limestone and marble are all carbonaceous bedrocks and are easily dissolved by carbonic acid produced by the mix of rainwater with atmospheric carbon dioxide. When these minerals fall out of solution, they often rearrange themselves in organized patterns, creating the characteristic crystalline cave formations of stalactites and stalagmites.
Ice
By weight, the second-most abundant crystals on Earth are in the polar ice caps, glaciers and permafrost. Solid water is crystalline and can take the form of snow, hail, plate ice, permafrost or glaciers depending on the conditions of its formation.
Biosphere
Crystals can be made by animals. Aragonite is synthesized by most mollusks to produce their protective shells. Vertebrate animals have evolved to produce hydroxylapatite, or bone mineral, instead. Hydroxylapatite makes up 50 percent of skeletal weight. Sometimes crystal production is pathogenic. Purine metabolism in animals can also result in the production of uric acid crystals. Unfortunately, high levels of uric acid crystals in humans are correlated with the disease gout. Calcium oxalate crystals sometimes aggregate in the kidney, forming painful kidney stones.