corundum

Pliny says, corundum was used in the form of a splinter fixed in an iron style.

The stones have been rubbed into shape apparently by corundum.

This is true whether it has reference to a grindstone, emery, corundum wheel, or a plain oil stone.

Corundum and pure emery are ores that are very rich in aluminum, containing about fifty-four per cent.

Corundum is the hardest substance known, next to the diamond.