Amalgam Tattoo

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Is a grey, blue or black area of discoloration on the oral mucosa of the mouth, typically on the gums of the lower jaw.
It is caused by entry of dental amalgam into the soft tissues, usually during the placement or removal of amalgam restorations.
It is common, painless, and benign ( not harmful), but it can be mistaken for melanoma.
They are common and are found in up to 1% of people in the general population

Diagnosis

An amalgam tattoo can be distinguished from other oral lesions because it does not change significantly in size or color, Some amalgam tattoos appear radio-opaque on radiographs (i.e. they show up on x-rays), although in many cases amalgam tattoos have no radiographic features since the responsible particle(s) of amalgam are very small even though clinically the area of discolored mucosa is much larger.
If necessary, the diagnosis can be confirmed by biopsy,

Treatment

No treatment is required since the lesion is entirely benign.
 

Reference

http://en.wikipedia.org/wiki/Amalgam_tattoo