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.

Gemstones are minerals that are cut and polished to make beautiful "stones" for jewelry and other adornments. Humans traditionally consider diamonds, rubies, sapphires, and emeralds to be the four "precious" gemstones, with all other gemstones called "semiprecious." Many gems are transparent, meaning they allow some or most of the incident light to pass through them. To make a stone sparkle, gemcutters cut and polish the facets, or flat outer surfaces, of a gemstone to boost the number of times a light beam will reflect internally before leaving the stone and reaching the eye. Some semiprecious stones, such as malachite and lapis lazuli (a rock containing the mineral lazurite), are opaque to light; gemcutters polish them to a high gloss, without facets. This shiny, rounded type of gemstone is called a cabochon. Corundum (ruby or sapphire) specimens that contain tiny fibers of other minerals also may be cut into cabochons to create a luminous "star" effect on the stone's surface.