Calcium oxalate (in archaic terminology, oxalate of lime) is a calcium salt of oxalic acid with the chemical formula CaC₂O₄. It forms hydrates CaC₂O₄·nH₂O, where n varies from 1 to 3. Anhydrous and all hydrated forms are colorless or white. The monohydrate CaC₂O₄·H₂O occurs naturally as the mineral whewellite, forming envelope-shaped crystals, known in plants as raphides. The two rarer hydrates are dihydrate CaC₂O₄·2H₂O, which occurs naturally as the mineral weddellite, and trihydrate CaC₂O₄·3H₂O, which occurs naturally as the mineral caoxite, are also recognized. Some foods have high quantities of calcium oxalates and can produce sores and numbing on ingestion and may even be fatal. Tribes with diets that depend highly on fruits and vegetables high in calcium oxalate, such as in Micronesia, reduce the level of it by boiling and cooking them. They are a constituent in 76% of human kidney stones. Calcium oxalate is also found in beerstone, a scale that forms on containers used in breweries.