Cystine in bone marrow. Light micrograph of crystals of cystine among blood cells in a sample of bone marrow. Cystine is an amino acid that can form crystals in urine and other body fluids when present in excess amounts. The presence of excess cystine can be caused by a genetic disorder known as cystinosis. It can be diagnosed by measuring the levels present in white blood cells. The white blood cells are the larger cells, stained purple. The pale blue cells are red blood cells. The cystine crystals are at centre.