particles of mercury can get into the gums or a wound in the mouth when getting fillings done or replaced. and all the definitions i read of mercury tattoos on the internet say that is a mercury tattoo and that it is an accidental accumulation of particles in the gum during dental work.
but, the kind i have always heard about are not accidental. dentists would inject mercury into the gum on purpose near a cavity they filled to prevent that cavity from going down into the root. mercury has also been used to fill root canals. and, yes, they can spread out over time and become more visable. the treatment is to cut them out of the gums.