A heavy metal was a metal with a specific gravity greater than 5.0, and can be poisonous. Examples include lead and mercury.
The planet Motherlode had rich deposits of heavy metals. In 2269, this deposits were being mined by miners. (TAS: "Mudd's Passion")
According to "Ethan", Romulans searching the planet Alpha Onias III were looking for heavy metals in the rocks. (TNG: "Future Imperfect")