I don't like the shape of that polygon..
looks like it was torn from paper....BUT it is less than
a mm across. [ I have it trapped on a piece of transparent
tape] To the unaided eye...it looks like a fly turd.
I was using paper towels to wipe the clay off..
have been taking pics of paper towel inks today..
but so far..no matches. the color dots on the paper towels
are too darn big.