Slate, Skarn, Marble, and Quartzite are Metamorphic Rocks.
BUT SKARN is the type of rock that does not belong in the same category as the others.
Slate metamorphosed from mudstone. Marble metamorphosed from limestone. Quartzite metamorphosed from sandstone. All these rocks retain their solid forms.
Skarn is a metasomatic rock composed of calc-silicate minerals. Skarn is formed at the contact zone between carbonate rocks and silicic magma. Skarn is often rich in hydrothermal ore minerals. Skarn as a metamorphic rock is dissolved under metasomatic process and hardens back into a rock.