With remarkable flatness and durability, plus a generous 4" wide by 10" long surface, this plate excels at lapping tools and truing sharpening stones. The 3/8" thick steel substrate is manufactured to a flatness tolerance of 0.0005" (that's five ten-thousandths of an inch!) over its entire working surface, and coated evenly with abrasive right to its edges. Fracture-resistant monocrystalline diamond particles are bonded to the plate with a unique process for exceptional durability.