Answer:
Index minerals are defined as those minerals that signifies how much the original or source rock has undergone metamorphism. It indicates the grade of metamorphism. These index minerals are formed under very high pressure and temperature condition that is commonly found to be present in the metamorphic rocks.
Some of the examples of index minerals are kyanite, sillimanite, garnet, chlorite, staurolite, and epidote.
Thus, the index minerals determine the grade of metamorphism.
