Schists are metamorphic rocks of intermediate grade characterized by medium to coarse crystals, mainly composed of quartz and micaceous minerals, such as biotite and muscovite. These minerals create a smooth texture, vitreous lustre, and aligned foliation from a dominant direction of stress. This is referred to as schistosity and is shown by the blue markers and red arrows. The Barrovian sequence describes the relative metamorphic intensity that shale undergoes to evolve into foliated metamorphic rocks. From low to high grade, shale first turns into slate, then phyllite, then schist, and finally gneiss. The increasing grade indicates increasing heat and pressure, causing compressive deformation and recrystallization. Schistosity is an indicator of the transition of a phyllite into schist by the less parallel consistency within its foliation (see Phyllite sample). There can be several descriptor classifications of schists depending on a dominant mineral, protolith, or the presence of index minerals. Index minerals typically occur as porphyroblasts in rocks that have been subject to regional metamorphism and they are characteristic to zones of certain metamorphic grades. In order of increasing grade, these include chlorite, biotite, garnet, staurolite, kyanite, sillimanite, and sillimanite present with K-feldspar. This sample is classified as a kyanite-zone porphyroblastic muscovite schist. Its composition is dominated by muscovite seen as its light, pearly texture and also contains garnet and kyanite porphyroblasts. Kyanite being the highest-grade index mineral present.