It wasn't that simple to find marble dust, at least not here on the east coast. Googling it found everything but, and the masonry yards, here, never heard of it. I finally got a reply from a quarry, Georgia Marble, who had me contact their subsiduary, Imerys at 888-277-9636. They in turn gave me the names and telephone numbers of some of their distributors in the northeast, and finally one is sending me 3 samples of different graininess. They call it Calcium Carbonate, but assured me that it's much harder than limestone. And the cost is around 50 cents per lb. versus over $7 per lb. for imported marble dust. Thanks for pointing me in the right direction. If I didn't google "marble sand", I might never have found Georgia Marble.