The intense heat in the interior of the earth is responsible for the formation of igneous rocks. Due to this heat the interior always remains extremely hot. When the magma coming out from the interior towards the crust solidifies within the crust or on the surface of the earth, 'igneous rocks' are formed. These rocks are darker bedause they contain more tron contents, while the rocks confaming acidic elements are less black or of ash coloured Then best examples are granite and basalt. Igneous rocks are the oldest rocks on the earth and so these are called Primary' rocks. Igneous rocks are very much solid. 'Granite' is a well-known example of intrusive type of rock. Basalt is also the same type of rock.
Minerals like gold, silver, nickel, copper, iron. mica, graphite, etc are available.