Surface plates are mainly made of granite and diabase, but there are also ceramic plates. Granite and diabase are igneous rocks, with the main difference that diabase is a denser material than the granite.
Granite (Italian granito, from the Medieval Latin grani’um "grainy (stone)", from the Latin gra’num 'grain'). The vast majority of granites are crystallised magma formed by the melting the Earth's crust rocks. Granite is the most common igneous deep rock in the continental crust of the earth. Granite is a dense and hard stone that comes in a variety of colours.
Diabase is an intrusive rock in the grey granite and can be found in southern Sweden Diabase has been stored under high pressure for more than 500 million years. The material is fine-grained and has a very high density, which makes it suitable for surface plates and other applications requiring high dimensional stability. The material has high resistance to both external abrasion and chemicals. Diabase shares many properties with granite and does not conduct power and is not magnetic.