To Marko and anyone else who'd like to do an experiment
:
Could you please take a cup or so of sand from your sandbed (or sump), preferably near or under your rock, put it in a jar with some tankwater and shake it? Post photos of what the jar and your sandbed looks like during this process. I'm guessing the jar water will turn a shade of brown, but that's only a guess.
Looking forward to seeing a whole whack of photos!