If you were to look at a pressure-temperature phase diagram of carbon, you would see that diamond is stable above about 25 kilobars at standard temperature.
I looked up "adamantine" in another minerology/gem book I have and it is defined as, "A hard, brilliant luster like that of diamond"! Such a luster is due to a high index of refraction. Other non-metallic lusters include vitreous (glass-like), pearly, greasy, silky, and resinous.