Take two headpins for the stem of the stud earring and use two concave halves of a barrel clasp to hold the glitter.
Remove the loop from the barrel clasp using pliers. It should come out quite easily and will leave a hole in the barrel where the head pin can be thread through.
Coat the head of the headpin with hot glue before quickly pulling it through a barrel half and making sure it is well stuck. This step is pretty important so make sure the headpin is not crooked or wobbly.
Lightly and carefully coat the inside of each barrel with gloss varnish or clear nail polish. I used Fimo gloss varnish.
Gently sprinkle on some glitter in the color of your choice. Fix up the edges with a needle or a headpin if you need to. Let it dry and then apply another light coat of varnish on top.
Finally, cut the stem to about 1cm in length. I then borrowed some earring backings from another pair of earrings...
...and Voila! From a distance, the glitter looks like a gemstone. :D