Nick Ridgway
It’s sold as “N-gauge ballast”, being single size stone marketed usually by Gaugemaster, among other sources, as I find that the stuff sold for 4mm is too coarse and far too Plasser &Theurer for me.
It’s washed into place with an old paintbrush using woodworking adhesive watered down to the same consistency as milk, and with some washing up liquid added in. Bit messy to do though very therapeutic.