Depending on the stone composition, urinary stones can be divided into six categories: calcium oxalate stones, calcium phosphate stones, uric acid (urate) stones, magnesium ammonium phosphate stones, cystine stones and purine stones. Most stones can be a mixture of two or more components. The characteristics of each stone are as follows: Calcium oxalate stones have acidic urine and are characterized by hard, unbreakable, rough, irregular, brownish color. Calcium phosphate stone urine is alkaline, characterized by friable, coarse, irregular, grayish, yellow or brown, and often caused by urinary tract infection and obstruction. Uric acid stones have persistently acidic urine and are characterized by hard, smooth, granular, yellow or brownish-red color, and abnormal uric acid metabolism. Magnesium ammonium phosphate stones are kidney infections, characterized by smooth, faceted or vertebral bodies, mostly associated with diet. Cystine stones are rare genetic disorders characterized by hard, smooth, waxy, yellowish to yellowish-brown color due to familial inheritance.