An isograd is a concept used in the study of metamorphic rocks.
An isograd is a theoretical surface comprising points all at the same metamorphic grade, and thus separates metamorphic zones whose rocks contain different index minerals.
The minerals present in a metamorphic rock are important because laboratory experiments at high pressures and temperatures have provided a lot of information on the pressure and temperature conditions under which certain metamorphic minerals form.
For example, with increasing temperature and pressure the first minerals to form from a shale are micas, particularly chlorite and biotite.
With increasing temperature and pressure garnet appears, and then kyanite (at relatively high pressure) or sillimanite (at relatively high temperature).