Geologists use the phrase metamorphic grade to describe the degree of metamorphism a rock has experienced. The higher the temperature and pressure the rock has experienced over time, the more its metamorphic grade increases. For example, metamorphism can change mudstone, originally a fine-grained sedimentary rock, into smooth-textured slate, which splits easily into thin sheets. Over time, the slate will become phyllite, a metamorphic rock that breaks into thick slabs. If metamorphism continues, phyllite turns into schist, which is considered a medium-grade metamorphic rock, and eventually into gneiss (pronounced "nice"), a high-grade metamorphic rock. Once the rock starts to melt under still higher temperatures, it becomes a migmatite, and when it melts completely, it turns into magma which ultimately hardens into igneous rock.