Begin with the blank pendant - mine came from that well-known auction site. I've foiled the back of these (just to see what the effect would be)
Add some solvent based glass paint - I used a pipette (this is Rainbow Glass because it's what I have handy, Pebeo Vitrail is very good also). I started with some clear to dilute the colours a bit as they are very concentrated. This time I've used green and blue.
The effect so far - you can see how the foil catches the light and reflects it through the transparent paint.