When evaluating diamond stones, the most critical factor is the grit size (measured in mesh or microns), which determines the finish quality. You must also verify the diamond concentration and distribution to ensure consistent performance. For sharpening applications, choose between monocrystalline diamonds, which offer superior longevity and durability, and polycrystalline diamonds, which provide a faster cut but wear down more quickly. Additionally, ensure the substrate material (usually steel or aluminum) is perfectly flat to maintain precision.