Cystine crystals are colorless, hexagonal plates that typically occur in congenital cystinuria - an autosomal recessive disorder in which renal reabsorption of cystine and other dibasic amino acids (lysine, arginine, ornithine) is impaired. Classic cystine crystals are thin and regular in shape, but this shape is often obscured when multiple crystals are superimposed. Usually this occurs in the same spatial plane, but they can also form three-dimensional aggregates. The crystals may appear loose, stacked or clumped together in groups, and under polarized light they are birefringent. They appear mainly in acidic urine (pH <5.5) and are a diagnostic indicator of cystinuria.