Calcium carbonate crystals are large circular crystals with radial striations with smooth surfaces. Smaller versions form pairs and tetrads. The crystals are slightly brown. Other forms are the dumb-bell, oval and tetrad forms, which are seen in the background. This is the most common urine crystal in equine urine and imparts a cloudiness and brown color to the urine (if in sufficient numbers). It is also seen in the urine of guinea pigs. The circular crystals can mimic ammonium biurate (which can have smooth borders), except that the latter crystals are seen in dogs and cats whereas calcium carbonate crystals have not been reported in the urine of dogs and cats.
In the background of this image, there are also numerous calcium oxalate crystals (hemp seed and oval variants).
