When a crown is removed, the underlying abutment tooth may appear charcoal-black. If the tooth is vital and free of decay, this discoloration is usually caused by some root canal sealants (especially older materials containing silver or gutta-percha) used during root canal treatment seeping into the dentin tubules and staining the tooth from the inside. A second cause is the reaction of metal corrosion products (copper, silver, nickel oxides) leaking from metal-supported crowns with the dentin tissue. If the tooth is vital and no metal has been used, this discoloration is caused by sulfur gases produced by anaerobic bacteria that grow inside as a result of microscopic leakage, chemically darkening the tooth tissue; this may be a sign of hidden decay.
