Calcium oxalate monohydrate crystals occur in various forms, such as oval, oval-rectangular, dumbbell, biconcave discs and elongated hexagonal structures. Their size can vary greatly: from very small to nearly field-of-view filling. They are usually colorless and show strong birefringence under polarized light.

The oval or round shapes may look like erythrocytes, but it is easy to distinguish between them using a polarizing filter.