When I built my first layout in N scale (back in the 1970s) I looked out for a cheap source of sand for ballast. What I found was so-called 'Bird Sand' - I think it is a decomposed granite sand with white, somewhat quartzy looking and black particles, giving the impression of a medium gray. At that time, 1 kilogram (about 2 lb) cost about $1.00. (The choice was clear: Compare this price to the apothecary prices for model railroad ballast in a LHS - also at that time there you got a small plastic bag with about 100g (3-1/3 oz.) for about $2.00 - about 20 times more expensive! :o )
I bought quite a reserve of this stuff, and so when I restarted model railroading a few years ago, I still found about 3 kg stashed away in the garage!
And so I use the same sand for H0 track, which was used before on N scale track.
Which brings us back to our basic theme in this thread: Scalewise I would say that this ballast is a bit too coarse for N scale - and perhaps it is a little too fine for H0. The average grain size is in the region of 1 millimeter (a little more than 1/32"). But I'll stay with it, because for me the perception of 'ballast' is just ok. But everyone sees it a little different, of course.
I am adding two pics from my old N and my new H0 layout, so you can judge for yourself. (BTW, I toned down the light gray a bit with a wash of inkahole (india ink & alcohol).
PS: A final remark: I went out to several supermarkets and looked again for bird sand - but I wasn't able to get Granite sand somewhere! Now you only get two much finer grades - one is buff colored and mixed with crushed seashells, and the other one is snow white and extremely fine. I use it (also toned down) to 'ballast' lowly sidings like the track in the foreground of my second pic. (And yes, the price went up to about $3 for a pound...
)
Ron