Granite Slabs and Tiles - Granite Wholesale - Chicago MGSI marble and granite supply, marble and granite supply of illinois, marble and granite, granite and marble, chicago marble and granite, marble & granite supply of illinois, marble and granite wholesale, marble and granite suppliers, marble granite suppliers, marble granite wholesale, granite marble wholesale, granite and marble suppliers Granite. Granite is a widely occurring group of intrusive, igneous rocks that form at great depth and pressure under continents. It is composed primarily of quartz, feldspar and mica.