Formalin pigment (acid formaldehyde hematin) is an iron-free derivative of hemoglobin that forms in skin when a specimen containing hemoglobin is fixed in acid formalin at pH less than 6. With low-power light microscopy, formalin pigment appears brown and granular and must be differentiated from melanin, hemosiderin, aposiderin, and hematoidin. Higher power reveals formalin pigment to be microcrystalline and birefringent. To avoid this artifact, tissue should be fixed in properly buffered formalin.
Ackerman AB, Penneys NS. Formalin Pigment in Skin. Arch Dermatol. 1970;102(3):318-321. doi:10.1001/archderm.1970.04000090080013