I think I answered that earlier or in another thread, but short & sweet. They are small stones that I get in a "pound" bag from a florist shop, who use them in floral pieces. They are a natural ?stone? grit product that tend to come in a great variaty of colours and size. (Size in the bag, colour options per bag

)
I just take a fistfull out, spread them on a piece of (news)paper and sort out the ones I want to use. Those are enbeded into the Elmer-glue/sand somposite I usualy use to fill of bases. And then they become part of the normal painting proces (or in some cases I just leave them as is)
Hopes this helps, if not PM me with the picture you mean...