Erythrite is a secondary mineral composed of hydrated cobalt arsenate, with the chemical formula Co₃(AsO₄)₂•8H₂O. It typically forms as a bright pink to crimson or purplish-red crystalline coating or radiating fibrous crust on the surfaces of weathered cobalt-bearing ore deposits.

Historically, prospectors would use erythrite as a sign for other minerals that would form in the same conditions as erythrite, such as cobalt and native silver.