These are harmless lesions caused by the inclusion of a piece of dental amalgam, or a tiny amount of amalgam dust under the oral mucosa due to some form of surgery or injury.  These are almost always flat and cannot be felt by the finger (unless the piece of amalgam is quite large).  Upon occasion, the amalgam scrap is visible on an x-ray film of the area.

AmalgamTattoo2Copyright 2006 Martin S. Spiller, D.M.D. courtesy of Dr. Ed Cataldo
Amalgam Tattoo on buccal (cheek) mucosa behind the second (last) molar.

LikenPlanusCopyright 2006 Martin S. Spiller, D.M.D. courtesy of Dr. Ed Cataldo

This is an image of liken planus, but it also shows an amalgam tattoo, which is the blue-gray splotch below and right of center. Amalgam tattoos can occur any place in the mouth where there has been a break in the mucosa during a procedure in which amalgam dust has been produced. This happens very commonly during the removal of old amalgams, or the preparations of teeth with amalgams for crowns. An errant bur stroke abrades the mucosa and amalgam dust is introduced into the wound. When it heals, the amalgam dust shows up through the healed mucosal surface. Amalgam tattoos are harmless, and no treatment is indicated.

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