Cystine crystals indicate an abnormality in metabolism of the amino acid cystine. These crystals appear as colorless, refractile, hexagonal plates with even sides. Cystine crystals sometimes occur in pairs. They produce a red color in the nitroprusside reaction. Cystine may be confused with the hexagonal forms of uric acid but does not polarize light. Cystine crystals occur in acid urine.