Originally Posted by cobalt327
The center two plugs (especially) seem to have some small spheres of aluminum. I hope that's not what it really is- I've "doctored" the images to make them a little easier to see.
My left to right is your top to bottom:
That's just fuel chemistry, if this was lean, or the cylinder in detonation/preignition, the insulator would be white (if not white, white) and if the spots were melting aluminum off the piston that would appear as dark splotches on the insulator.