Cubic zirconia! Almost a diamond, but not quite!
It won't because diamond, the gemstone, already has a dragon. Just like ruby, emerald and sapphire. They may make other gemstone dragons, but diamond is already claimed. Personally, it would be nice to see dragons of many gemstones that can be crafted, but of course, they would be limited to the colors of the elements available. (red, blue, green, yellow, purple, pink, white).
Coloured diamonds!
My birth stone is pearl, so ot better be a pretty decent dragon...