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The name comes from a type of shell (latin - perna) or spherical shape (Latin - spherula). Pearls are made by mollusks and rarely snails ... They are composed of "mother-of-pearl, which is mostly calcium carbonate (in form of aragonite), and an organic horn substance (conchiolin) which form concentrically around microcrystals. Pearls are very dense and it is difficult to crush them even though their Mohs' harness is 3 or 4.