Whewellite

Whewellite is a mineral, hydrated calcium oxalate, formula CaC₂O₄·H₂O. Because of its organic content it is thought to have an indirect biological origin and this is supported by it being found in coal and sedimentary nodules. However, it has also been found in hydrothermal deposits where a biological source appears improbable. For this reason it can be classed as true mineral.