As shvrdavid said, you can adapt the shader to emit light.

If you really want a light inside the gem, load a point light. Shrink it to be smaller than the gem. use the front view camera to line it up with the gem from that angle. Then use the side (left or right) view camera to line it up from that angle. You might want to set the gem to outline or wireframe so you can see the light easier. I also often use frame selected object (with the light selected) to get a close-up of it. Then parent the light to the gem to move them around.

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