Calcium carbonate - Halter
Synonyms: Aragonite, Calcite, CaCO₃
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 dumbbell shape is mainly found in highly alkaline urine.