Explanatory notes
… or building stone Granite is a very hard, granular igneous rock formed by the agglomeration of quartz crystals … also includes other hard igneous rocks (e.g., syenite, gneiss, trachyte, lava, diabase, diorite, phonolite), as … marble (or ophite) which, being a natural form of magnesium silicate, cannot be classified in heading 25.15 . …