The Burton line or Burtonian line is a clinical sign found in patients with chronic lead poisoning. It is a very thin, black-blue line visible along the margin of the gums, at the base of the teeth.
The sign was described in 1840 by Henry Burton:
The edges of the gums attached to the necks of two or more teeth of either jaw, were distinctly bordered by a narrow leaden-blue line, about the one-twentieth part of an inch in width, whilst the substance of the gum apparently retained its ordinary colour and condition.
A similar line, the "bismuth line", occurs in people who have ingested bismuth compounds; bismuth, however, is of very low toxicity.
References
Burton line Wikipedia(Text) CC BY-SA