Calcium carbonate crystals are rarely found in human urine, but occur in alkaline environments. They most often appear as small, colorless, dumbbell-shaped structures that lie in clusters and can therefore be easily mistaken for bacteria or fungi. Under polarized light, they are usually not birefringent. Calcium carbonate crystals are more commonly seen in herbivores (such as horses), and their clinical significance in humans is still unclear.

This round shape with (sometimes) radial striations can visually closely resemble leucine crystals, especially when bilirubin is present. However, leucine crystals are yellow-brown in color, show a characteristic radial or concentric pattern and are seen in severe liver diseases such as liver cirrhosis or tyrosinemia.