5/16 8.1 mm DiamondSure Diamond Drill Bit for Glass, Tile, Ceramic, Porcelain, Stone Model: 900-004 Bonded Diamond Grit: Electroplate bonded diamonds for optimum diamond life. Nickel Plated: Reduces bit friction and heat build-up and inhibits rust. Grooved Tip Edge: Provides maximum surface area & increases water churn for tip lubrication. Tip Lubrication Hole: Provides optimum tip lubrication by allowing water flow in and out of bit. Core Extraction Hole: Special holes in back of the Bell allow easy removal of cores or plugs.