Oxalis leaf. Coloured scanning electron micrograph (SEM) of a freeze-fracture through an oxalis leaf to show calcium oxalate crystals (orange). Calcium oxalate is a common biomineral in plants, occurring as crystals of various shapes. It can be found in any tissue or organ in plants and is often formed in the vacuoles of specialized cells called crystal idioblasts. Recent studies indicates that calcium oxalate formation is generally a mechanism for regulating bulk-free calcium levels in tissues and organs. However, various other functions might have evolved secondarily. A function in physical protection against grazing animals is implicated by the size, shape and placement of crystals in some tissues and organs. Magnification: x100 when printed at 10 centimetres wide.