Slate is composed mainly of clay minerals or micas, depending upon the degree of metamorphism to which it has been subjected. Slate can also contain abundant quartz and small amounts of feldspar, calcite, pyrite, hematite, and other minerals. The original clay minerals in shale alter to micas with increasing levels of heat and pressure. Slate is formed when a sedimentary rock (shale, mudstone, or basalt) is compressed. You've likely encountered slate on a building or an old chalkboard. The most common color of slate is gray, but it can also be brown, green, purple, or blue. Slate is a metamorphic rock with a dull luster. Slate, fine-grained, clayey metamorphic rock that cleaves, or splits, readily into thin slabs having great tensile strength and durability; some other rocks that occur in thin beds are improperly called slate because they can be used for roofing and similar purposes. Over time, slate may transition into other metamorphic rocks, such as phyllite or schist.