| Summary: | Crystals are very common and, aside from regions of its molten interior, the Earth can be considered to be a crystalline planet with many types of crystals, expressed as thousands of mineral varieties, occurring in diverse environments. Some of these crystals (and minerals) are of geoheritage significance because of their size, composition, shape, zoning, reaction rims, mineral inclusions, fluid and gas inclusions, twinning, dislocations, exsolution, the unusual occurrence or rarity of a crystal attribute, and their form of aggregation, such as desert roses, or druses. If macroscale geological features are assessed as important in unravelling Earth history, and afforded geoheritage significance, then importance should also be given to crystals where similar principles and patterns are present. Some notable crystals of geoheritage significance are the giant gypsum in Mexico and in Spain, the zircon crystals of Jack Hills (the oldest crystals on Earth), the large well-formed pyrite in Spain, snowball garnets from various locations, and Iceland spar from its type locality.
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