Corundum
Corundum is the hardest mineral other than diamond. Ruby and sapphire are the most valuable corundum. Red corundum is ruby, other colors are called sapphires, but there is no clear border between the two. Light red, pink or violet corundums are usually classified as sapphires whereas ruby is reserved for truly red stones. Corundum is the crystalline form of aluminium oxide. It is naturally white or clear, but can have different colors when impurities are added. Although Corundum is very hard, it is only 1/140 as hard as diamond. Because of its hardness, large quantities of fine-grained inexpensive corundum are mined industrially. Industrial grade corundum is used to manufacture abrasives (emery and sand paper) as well grinding and polishing powders. Synthetic Corundum
Gem stone quality corundum can be produced by various methods (flame-fusion, hydrothermal, flux-growth). The DeLong Star Ruby is a 100 carat ruby discovered in Burma in the early twentieth century. It was eventually donated to the American Museum of Natural History in New York in 1937, then stolen and recovered in a phone booth in Florida
The process is rather simple and it has allowed the production of large quantities of sapphires, rubies and other gems at a very affordable price. Synthetic rubis and sapphires are also used in the industry to make lasers, bearings, rods, tubes and other machined parts.
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