Sapphires are so hard because they are formed from corundum, an aluminium oxide.

So sapphire is 'dirty' corundum - corundum with a trace of iron and titanium.

In its natural state, it is the mineral corundum.

Herein also is the kind of corundum known as emery, and esteemed for its polishing properties.

Oats and corundum both came according to prophecy.

What is said of corundum and mica?

And I should not be surprised if corundum could be found in those rocks back of the house.

It was near the valley that Moynglass found his sample of corundum, with a ruby crystal in it.

Diamonds are pure carbon, but did you know that rubies and sapphires are corundum minerals-rareforms of alumina.

The amount of raw corundum used annually by the company reaches nearly 1,000 tons.

They're actually a synthetic corundum.

Of some of these minerals, as corundum and mica, North Carolina has already become the chief source of supply.

Aluminium oxide or alumina, Al2O3, occurs in nature as the mineral corundum (q.v.

The exhibit showed corundum in bulk, in large wheels, small wheels, hones, and every variety of grinding and sharpening specialties.

There is no known substance, not even corundum, hard enough to resist the swift impact of myriads of little stones.

It was mined in Sri Lanka's central district of Ratnapura and is a classified as a natural corundum un-heated blue star sapphire.