It is black or dark grey in colour, less dense than translucent-brown corundum with a specific gravity of between 3.5 and 3.8.
en.wikipedia.org It occurs with calcite, ettringite, wollastonite, larnite, brownmillerite, gehlenite, diopside, pyrrhotite, grossular, spinel, afwillite, jennite, portlandite, jasmundite, melilite, kalsilite and corundum in the limestone xenoliths.
en.wikipedia.org For example, ruby is the red variety of the species corundum, while any other color of corundum is considered sapphire.
en.wikipedia.org Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisite, dolomite, corundum, and, rarely, diamond.
en.wikipedia.org Bauxite is a very common ore which, along with corundum's reasonably high hardness, contributes to corundum's status as a common, inexpensive abrasive.
en.wikipedia.org