Final answer:
Garnet is the index mineral for higher metamorphic grades among the options given. It is indicative of medium- to high-grade metamorphism and can be found in rocks like gneiss.
Step-by-step explanation:
Among the options provided - Calcite, Garnet, Clay, and Quartz - Garnet is considered an index mineral for higher metamorphic grades. The concept of index minerals is crucial for geologists as they indicate the conditions under which a rock metamorphosed. Rocks with a shale protolith will have quartz, feldspar, and muscovite mica at all metamorphic grades, but the appearance of new minerals, particularly seen as porphyroblasts, indicates the level of metamorphic grade.
Garnet is a silicate mineral that forms under the high temperatures and pressures associated with medium- to high-grade metamorphism, as illustrated by a rock like Gneiss, a high-grade metamorphic rock. The presence of garnet, particularly in rocks where it's not common, serves as a robust indicator of the high degree of metamorphism. Other minerals indicating specific metamorphic conditions include kyanite, sillimanite, staurolite, and cordierite.