Heavy metals are a sub-group of metals. However, there is no clear, scientifically accepted definition of the term. Various sources classify those metals as heavy metals, whose density exceeds 5 (in older works 4.5) grams per cubic centimeter. Heavy metals occur in the rocks of the earth's crust, where they are firmly incorporated into ores as oxides, sulfides and carbonates, or are also confined in silicates. Some of them can be found in pure form. Their concentration in the hydrosphere, atmosphere and pedosphere covers an area of several orders of magnitude. Weathering and erosion cause heavy metals to penetrate naturally into soils and groundwater. Some rocks such as picrite basalt, serpentinite, basalt and especially ores contain high concentrations of chromium, nickel and cobalt. The surrounding soils are thus exposed to a high natural heavy metal contamination.
In the course of industrialization, the amount of heavy metals present in the material cycle and the accumulated amount of heavy metals in the environment have also risen sharply. The reason for this are increasing emissions from various anthropogenic sources, which include the extraction of heavy metals and their processing as well as the production of fertilizers, the combustion of coal, the incineration of waste and sewage sludge, motor traffic and the production of steel, cement and glass. In particular, the mining of heavy metal ores is often accompanied by a high heavy metal contamination of soils. An azonal vegetation of specific plant communities has developed on the soils that were contaminated by ore mining in some places in the Harz, in the Siegerland and in the environment of Aix-la-Chapelle. Calamine plants generate the so-called heavy metal turf.