Step 2: Prepare the marbles
The marbles will also need a small patch of roughed up area for contact. Again I used the Dremel and an abrasive wheel to grind a small flat on the surface.
The flat of the LED is glued to the ground spot on the marble using Cyano type super glue. This glue has the added benefit of filling all of the grind marks and making an optically pure connection. The last pic clearly shows the LED on the other side of the marble.