Methenamine silver stains urates black. Sodium urate crystals are also birefringent on polarization. At 90 degrees to this, the crystals will be blue. Uric acid crystals, polarized, with red plate. These come from industrial or environmental exposure by inhalation, ingestion, or contact.
Carbon appears as anthracotic pigment in the lungs.
It can be distinguished from melanin by doing a melanin bleach. Poorly fixed tissues may contain formalin-heme pigment, which is black and finely granular, but this is widely scattered in the tissues without regard to cellular detail. Formalin-heme pigment is also birefringent on polarization. Asbestos is a special type of long-thin silica crystal, usually of the mineral group chrysotile. In tissue, these crystals are highly irritative and highly fibrogenic.