A pigment derived from hemoglobin that contains no iron but is closely related to or identical to bilirubin. Hematoidin is formed intracellularly, presumably within reticuloendothelial cells, but is often found extracellularly after 5-7 days in foci of previous hemorrhage. It occurs as refractile, yellow-brown and orange-red granules, but more characteristically as rhomboid plates arranged in a radial pattern, so-called hematoidin burrs. SYN: blood crystals, hematoidin crystals. [hemato- + G. eidos, resemblance, + -in]