What is "adamantine spar"

Corundum \Co*run"dum\ (k[-o]*r[u^]n"d[u^]m), n.; pl. Corundums (k[-o]*r[u^]n"d[u^]mz). [Also corindon.] [From Hind. kurand corundum stone.] (Min.) The mineral alumina ( Al2O3), as found native in a crystalline state. Transparent varieties are used as gemstones, including sapphire, which is the fine blue variety; the oriental ruby, or red sapphire; the oriental amethyst, or purple sapphire; and adamantine spar, the hair-brown variety. It is the hardest substance found native, next to the diamond.

Note: The name corundum is sometimes restricted to the non-transparent or coarser kinds. Emery is a dark-colored granular variety, usually admixed with magnetic iron ore.

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.