Contaminated stone from W/D?

How do I tell if there's grit contamination in my stone?

I've flattened using W/D (~400grit IIRC). My Naniwa has "spots" on the surface (fairly large spots ~2-3mm dia) that reading elsewhere is supposedly "normal" after flattening
I don't feel anything with my hand and have rinsed the stone quite a number of times now.

I think they use a coloring in the sandpaper to make it look good and the paper dye transfers to the stone. Shouldn't hurt anything. If its a 12K SS, it will discolor very easily.

I've got an 8k and 10k. Yellow and green. That would make sense if it's dye.

Any idea how I could tell if I have embedded grit?

SpikedLemon said:

Any idea how I could tell if I have embedded grit?

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Pass a highly refined edge over the spot in question, then view under scope to see if the edge has degraded.

Scrub it with a nylon fingernail brush under running water.

Naniwa Super Stones have spots - like freckles almost. Some larger than others. That's normal.