Well prepared strength test specimens and components break from a material s inherent flaws and machining damage has no effect on strength. On the other hand, grinding can create machining flaws that control strength and limit the performance of finished ceramics. Machining cracks sometimes are difficult or impossible to detect and characterize, especially in toughened ceramics. Machining cracks sometimes are difficult or impossible to detect and characterize, especially in toughened ceramics with interlocking grain microstructures that create rough fracture surfaces. New fractographic examinations have shown that machining damage leaves telltale markings on fracture surfaces that may be easily detected using common fractographic techniques. A comprehensive study with over 400 ground rods and rectangular bars was conducted on several commercial silicon nitrides to study the effects of various machining conditions. Similarities and differences in behavior were observed. A paradoxical finding that was tougher silicon nitrides developed deeper grinding cracks. Machining crack size and shape strongly depended on the grinding wheel grit size.