A diagnostic mineral in a regional metamorphic terrain whose first appearance, going in the direction of increasing metamorphic grade through the metamorphic sequence, marks the outer limit of a metamorphic zone. A line marking the first appearance of an index mineral is termed an ‘isograd’, and represents a line of constant metamorphic grade. To indicate the regional distribution of metamorphic grade the first appearance of index minerals in rocks of pelitic composition (i.e. shales) are usually mapped out in the field, as this type of rock is extremely sensitive mineralogically to changes in metamorphic grade.