Bright, as in shiny? Or bright as in light (like a lamp)?
If shiny, I would go with the light through the diamond, the diamond itself, and the metal band.
The diamond is so bright because of the way its cut. With all the different sides, the light that enters it from above gets tossed around, bouncing off all the different sides before exiting it. This makes the diamond look bright and shiny.
The light through a diamond, how do I explain this. It's like when light is shining through a window pane with shapes in it. Certain parts create a rainbow on the wall or floor behind the window. This is because of the refraction of the light as it goes through the diamond. It lights up brightly where the diamond would reflect it.
For the band, the metal is naturally bright. It reflects the light in a shiny way from the moment its extracted from the stone it formed in. When they form it into a ring shape, the ring maker buffers it as well, giving it a bright shine on the outer surface.
I really hope you meant a ring like an engagement ring, otherwise this answer will be of no help to you.
