Adamantine is the term used to describe the luster exhibited by diamonds and gems with a Refractive Index of 1.9 – 2.5. The root word adamant derives from the Greek word adamastos which means untameable. Throughout history the word adamant has been used in reference to anything that was composed of a particularly hard material, diamond or otherwise.
Subadamantine luster is slightly less than that of a diamond. Example: Zircon.