The chemical term earths was historically applied to certain chemical substances, once thought to be elements, and this name was borrowed from one of the four classical elements of Plato. "Earths" later turned out to be chemical compounds, albeit difficult to decompose, such as rare earths and alkaline earths.

In fact, earths are metallic oxides, and the corresponding metals were clasified into the corresponding groups: rare earth metals and alkaline earth metals.

Inorganic chemistry