diamond drills are used on glass, stained glass, ceramics, ceramic tile, porcelain, porcelain tile, limestone, marble, granite, slate, stone and fiberglass. Drills used on wood or metal have a sharp metal tip or teeth, that cut into the material. These types of drills do not work on glass, marble, etc. As the tips do not "bite" into the extremely hard material, and cause heat buildup that burn up the bit and cause "heat fractures" in the material. Carbide tipped spear point drills are sometimes used on the "softer" types of hard materials - like untempered glass, sandstone and marble and some of the less hard ceramic and porcelain tiles. While they sometimes work, they tend to chip the hard surface very badly, leaving a rough hole and often cause breakage due to fractures from the chipping and heat. Diamond drill bits are designed differently - they have diamond tips that "grind" into the extremely hard materials.