Cystine crystals are pathologically crystalline, colorless, hexagonal, well-defined, refractive lamellar crystals, often in overlapping arrangements. Cystine is derived from the decomposition of proteins and is rare in urinary precipitates. It is insoluble in acetic acid but soluble in hydrochloric acid, can be rapidly dissolved in ammonia, and may reappear upon addition of acetic acid. The cystine test may show a blue or green reaction, and the clinical significance of the presence of cystine crystals in the urinary sediment is renal stones and bladder stones. Cystine crystals are relatively stable at room temperature and pressure, and are found in nature mainly in tobacco leaves, but also in animal hair and bone. There are three ways to synthesize cystine crystals: oxidation of an alkaline aqueous solution of L-cysteine by air, extraction from hair, and production of tyrosine as a by-product. The extraction from hair is less common, with only five percent of the crystals actually being produced. Cystine crystals have a wide range of uses, it is mainly used in biochemical research, making biological culture media, etc.. It is also used for conditions such as alopecia areata and as a food additive.