Well, isn't it interesting - lol. I also Googled "bulk pumice stone", and sure enough it looks just like Matrix. So then I Googled "Matrix pumice" and found another forum conversation where the same thing was said. That certainly explains the dust! Plus they both float - another giveaway.
So I have 2 questions; (sorry for the thread hijack!) Plain old pumice from a garden center should be aquarium safe if there are no additives listed on the bag, right? Also, since it's pores will harbor nitrifying and denitrifying bacteria, why can't it be used in the substrate? Like placing a layer under MGOCPM, or Black Diamond? If whatever is over the pumice is a thick enough layer, the pumice still wouldn't show after all the smaller particles of the other material fell though the gaps. Or would that be so deep anaerobic problems could start? Just a thought for increasing nitrifying and denitrifying surface area, I don't see why it has to be in a filter to work...(my apologies again, OP, for the derailment)