Nepi get the regular plaster of Paris from Lows or Home Depot, get the stones from a bead store, it is good if they are small like the kind you would use on a bracelet , make the plaster with a little more water than the called for proportion (2 plaster and 1 water) put 1/3 of the plaster in the mold, put in some stones, another third, a bit more stones, and the rest with all the stones left.
Use in it a little oxides that you have, chromium oxides, copper powder, or copper sulfate, the copper powder After a week of the plaster drying, you can paint it with some shellac at that point your device will be turning a wonderful shade of light green.
More to come
Josh