mica-sized crystals. as the temperature and pressure increase further, we develop a schist. this is a garnet schist with large garnet crystals. lots of white mica. very coarse crystals. this would be formed at quite high metamorphic grade. at a higher metamorphic grade that you constitute a gneiss where you start having minerals segregate into definite layers. if metamorphic rocks form inside the earth as temperatures and pressures rise, why aren't they unmetamorphosed as temperatures and pressures fall back down? in part, this is because loss of fluids during metamorphism makes it impossible for certain chemical reactions to reverse themselves. also, as temperatures drop, ions cannot migrate easily through the rock, so minerals will not recrystallize. so, in most metamorphic rocks, geologists find a preserved record of the greatest temperatures and pressures occurring during crustal deformation. with the development of plate tectonics theory, the temperature and pressure changes geologists have long seen in metamorphic rocks, finally began to make sense. for many years, geo