If you want to go SUPER fine, Highball makes Z scale ballast.http://www.modeltrainstuff.com/SearchResults.asp?Search=highball
I have 240 yds of track, so needed a cheap form of ballast for my HO layout. I use "Kitty Litter" that has been ground up in an old coffee grinder. Some others suggest aquarium sand (as is, or painted to the desired color). Bob Hahn
Prototype ballast stone measures about 3-4", so calculating scale compression etc etc it comes out to about "fine" grit ballast for HO. Medium for O, chicken grit for G. N scale would be "dust", but use whatever comes under "fine" for the brand you are using OR you could make your own from sawdust and dye it to the color of your favorite railroad's stone.
The Dixie D Short Line "Lux Lucet In Tenebris Nihil Igitur Mors Est Ad Nos 2001"
I'm using fine or N scale for HO. I think it looks more accurate.
I am at that wonderful stage in model railroading called ballast. I started using medium size ballast but looking at it, it seems alittle too big. I was wondering if anyone used the fine size ballast and which looks more realistic?