There are also manufactured gems. Early in the 19th century several experimenters successfully produced artificial reproductions of rare stones, such as the ruby. In 1902 Auguste Verneuil of Paris established the first commercially successful process for making such synthetic gems. The chief raw material is extremely pure, powdered aluminum. This is sifted through an intense oxyhydrogen flame, and the fused material drops down on a sticklike support.
Pure alumina yields white sapphire.
Mixed with a small amount of chromium oxide, it makes synthetic ruby. Other mixtures yield blue, green, rose, or violet sapphires. A variation of the Verneuil process, developed in the United States in World War II, produces slender rods up to 30 inches long. They are especially useful in the manufacture of jeweled bearings for precision instruments. Synthetic gems are used principally in industry.