Hi Terence,

The rock dust should be about 0.1mm or finer for optimal surface area. Basically like a finer powder. The larger the pieces, the more you need for equivalent surface area.

There are actually quite a few companies around the world griding rock for this purpose and it definitely works.

You might even find some in a local organic gardening center. You can buy it all over the net. Might have spendy shipping costs depending on how much you get.

One cheap and easy source might be to contact a local gravel pit company and see if you can get the dust from the cement kilns. There is a good cross section of minerals in that and that dust will work just fine. Maybe not the best, but there is benefit.