It is a pretty simple process. You fit rods, squares, triangles, etc. in the tube and suck epoxy up the tube. Let the epoxy cure and you have a mosaic rivet.
For fancier rivets, you make shaped inserts and draw up the epoxy. If you have a bi-directional initial (A,H, I, O, T,X), you can make a rivet with your initial in it. I do this with an "A".