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.
Diamonds capture and radiate the light which makes it sparkle. You can always tell if the diamond is real if it has air holes underneath it and it always sparkles different colors if it ever reflects any black or dark color the diamond is most likely not real. Hope this helps :)
Diamond will only sparkle when there is light.