Synthetic ruby. Cooled sample of molten aluminium oxide powder used to make a synthetic ruby in the Verneuil process. Rubies are a form of the naturally occurring mineral corundum, an aluminium oxide. The Verneuil process uses powdered alumina (aluminium oxide) that is heated to 2000 degrees celsius in a furnace. This melts the powder, which forms a boule as it cools on a supporting rod. The colour is obtained by adding chromium oxide for rubies, and ferric oxide for sapphires. The process produces gems that are chemically and physically equivalent to their naturally occurring counterparts.