This page has images of spark plugs for tuning purposes.
This plug shows light peppering of the porcelain due to
detonation. The tiny black specks are what we are looking for. You may
or may not be able to make out that some of the specks are
purple in color. This is from fuel additives.
plug showing a light peppering from detonation. The tan area to the
left is from the incoming fuel, that side was toward the intake valve.
More peppering on this plug from detonation. Along with purple splotches from fuel additives.
The above three plugs came from the same engine. The bone white porcelain centers is from running in closed loop.
plug from an engine running open loop. No peppering is present. The
dark splotches is from oil being pulled through the PCV system.
Another spark plug from same engine (out of focus, will get some other pictures at a later date).