Fiber reinforced means they will not break easily, even when bent. As compared to other extra thin separating discs, Micro Fiber Discs have superior cutting ability and are more economical because they are more durable. Can be used up to 50,000 rpm while generating minimal heat. Available in 2 sizes and in 2 types: aluminum oxide for cutting most metal alloys and silicon carbide for pure gold, zirconia, and porcelain. The 22 mm (about 7/8") diameter discs are 0.3 mm (about 0.012") thick. The 35 mm (about 1 3/8") diameter discs are 0.7 mm (about 0.028") thick. Provided in packages of 10 pieces. All discs have a hole diameter of 1/16".