Calcium oxalate crystals in the urine are the most common constituent of human
kidney stones. Also, blood in the urine can be caused by a lot of things. Some are serious, including cancers, trauma, stones, infections, and obstructions of the urinary tract. Others are less important and may require no treatment. These may include viral infections, nonspecific inflammations of the kidney, medications and benign prostate enlargement.
As kidney stone develops, they can also cause hematuria.