Having heard good things about pumice on this list, I went to our OSH and purchased a small bag of Black Gold brand pumice. It was about 10 times heavier than I expected pumice to be, but they assured me that was just because it was wet. At home, I noticed it had a lot of dust, which I did not think I wanted in my growing medium (not having read the note from Jane yet about the dust containing useful minerals). When I washed the pumice, I noticed it sank. I expected pumice to float. The pumice in Baja California floats and resembles styrofoam. I wrote to Black Gold and they replied "The Black Gold Pumice that you bought comes from a source in central Oregon, the same source we have used for a long time. Chances are that you will get the same type of pumice in California, Oregon and Washington, if you buy Black Gold. This is a denser pumice, but should not affect the porosity you are seeking. The dust comes from the pumice rubbing against itself in the packaging as it is moved around." I also noticed when I washed it that I could sieve out the sizes to get pebbles for topping and grit to sand size for growing medium and could wash away the silt-sized sediment. - Gastil