Rocks are grouped according to

Rocks are grouped according to their formation into three major groups - igneous, sedimentary, and metamorphic rocks

Explanation:

Based on their formation, rocks are further sub-grouped as: Igneous - Intrusive or extrusive Sedimentary - Clastic, chemical, or biologic rocks Metamorphic - Foliated and non-foliated Rocks are also grouped based on their properties like size, shape, texture, chemical composition, permeability, mineralogy, etc. Based on properties rocks are sub-grouped as: Igneous rocks - coarse or fine-grained; glassy or obsidian; felsic or mafic. Sedimentary rocks - conglomerate, sandstone, breccia, shale, siltstone, chalk, crystalline or fossiliferous limestone, gypsum, rock salt etc. Metamorphic rocks - Gneiss, hornfels, mylonites, cataclasties, marble, etc.