Product description. Set of drill bits for glass and ceramics by Sthor 21709. Designed for drilling precise and clean holes without edge chipping in glass, ceramics, porcelain. - Made of high-grade YG6X steel - blade with a hardness of 90 HRC - body with a hardness of 38 HRC - durable and resistant to damage and dulling. Package contents. Drill bits: 4, 5, 6, 8, 10 MM Parameters. Diameter [mm]: 4, 5, 6, 8, 10. Material: YG6X. Handle: Hex.