Dwarven Earthspeak is a form of innate magic that every dwarf can perform. Since they are beings of the earth and are closely connected to Morum, they have supreme mastery of all of the rocks, stone, and soils of Verden.
Similarly to sorcery, most of the Earthspeak is performed by physically speaking words to whatever the dwarf wants to change. By doing so, dwarves are able to shape earth like clay, pass through it as if it were water, and imbue it with magic. Apparently, this only works on natural formations of these substances - once a stone has been removed from the earth, it is no longer part of it and so cannot be manipulated with Earthspeak.
One potentially negative side of their strong connection to the earth is that dwarves feel physically ill when near areas that the earth has been abused or corrupted. If they are too overwhelmed, they are forced into the Sunkensleep, where their bodies merge into the stone and they slumber for years or even centuries.