If you have no light, it will never sparkle like in a real black room. Diamond gives back the light it catches. Diamond has ability to catch light from the side, because its high refractive index. For a glass cut in diamond shape, it will need more light to sparkle like a diamond. Some diamonds can appear blue (generally) in a sark room, but with UV light much like a glass of tonic in a discotheque. This is called fluorescence, but not all diamonds fluoresce. A diamond with strong fluorescence will be less valuable than a similar siamond with none.
