Back to sand: I know that people are often looking for handy sources of this stuff from landscapers and masoners supplies. I just found a new source.
My son works in a roofing shop while making money for college. He brought me several small bags of finely crushed rock/sand in HO dimensions. It is used by roofing companies in their torch-on work on flat roofs. The stuff is sprinkled on tarry margins where the roll of roofing does not extend properly.
It looked like it just came out of a hobby store. There is a HO sized grey blend that looks like ballast, a reddish brick one, and a charcoal-coloured one flecked with the brick red.
It has some iron particles in it, but sure looks clean and sterile. It is dustless.
I like that it is anglar and not rounded looking like most other scenics rock material.
I am going to use some as is, in spots far from the tracks for now. It looks like running a magnet through it would make it perfect for many uses.
I know the roofing shops get this stuff in very large bucketfuls. I don't know what they might charge. Like other true rock stuff on your layout, it would be heavy in large quantities.