Calcium oxalate crystals are hard, crystalline mineral formations in the kidney or urinary tract
Urine contains many dissolved minerals and salts. Calcium oxalate crystals can form when your urine has high levels of these minerals and salts. Calcium oxalate crystals can start small but can grow larger in size, even filling the inner hollow structures of the kidney.
Some crystals stay in the kidney, and do not cause any problems. Sometimes calcium oxalate crystals can travel down the ureter, the tube between the kidney and the bladder. If the crystal reaches the bladder, it can be passed out of the body in urine. If the crystal becomes lodged in the ureter, it blocks the urine flow from that kidney.