Adamantine is a mineral, often referred to as adamantine spar. It is a silky brown form of corundum. It has a Mohs rating of 9.
Adamantine is also used as an adjective to refer to non-metallic, brilliant light reflecting and transmitting properties, known as adamantine luster. Diamond is the best known material to be described as having adamantine luster, although anglesite, cerussite and corundum in some of its forms are also described in this way.
References
Adamantine Spar Wikipedia(Text) CC BY-SA