A hollow quartz crystal you can attach to a flashlight. Use translucent or glow material, unless you just want a model of a crystal on a round base.
Done with 4 perimeters and 0 top/bottom layers. No problems with overhang. Used Hatchbox glow ABS. Fits my Garrity flashlight which has a 30mm ring around the top that it fits on. Can be scaled for other lights if they have a nice flat ring on the light end to work with. Use test ring to gauge how much to scale for other lights, it is the same ring used in the model, so if you can scale it to fit your light, apply the same scale value to the actual object. I scaled this by a factor of 1.01428 to get a perfect fit. Still works well as a flashlight, but light is diffused, glow effect is just an added bonus. My Rostock is actually a v3.