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. Silica is present in many minerals and building materials. Most forms are very inert and cannot be stained in tissue but can be demonstrated by white birefringence on polarization. It is most often present in lung, but can make its way into lymph node. Street drugs for injection often are diluted with compounds containing minerals such as silica or talc.