Detonation is something that should not be ignored. Severe damage can occur to your engine. When you pull a plug and notice small black or gray spots on the nose of the spark plug (looks like pepper), your engine is experiencing light detonation. In cases of severe detonation, insulators may be cracked and/or chipped. Detonation can break spark plugs, damage intake valves, and be the demise of pistons. Make sure the correct octane fuel is being used and ensure the correct heat range of spark plug is being used.
Possible causes: Improper octane usage or timing improperly set. Also, a spark plug with the wrong required heat range will allow detonation.