Synthetic gems have the same chemical, optical, and physical properties of their natural counterparts, but are a more cost-effective alternative to a natural gem. The molten powder repeatedly falls from the flame onto a rotating pedestal, creating a synthetic crystal, called a boule, which can later be faceted into a gemstone.
Other colored corundums are called sapphires, which come in many colors, the most well known being blue. The only difference between a ruby and a sapphire is simply the color. Powdered chemicals (the building blocks of the gem) are dropped through a high-temperature flame. The sapphire is a corundum, an aluminum oxide with a trigonal crystal structure, in the same family as the ruby. The flame fusion process for creating gems, also called the Verneuil process, is the most affordable and common synthesis method for producing corundum (ruby and sapphire) and spinel. When two or more intersecting bands appear, a star pattern is formed. Light that strikes the inclusions within the gem reflects off of the inclusions, creating a narrow band of light. Synthetic star sapphire exhibits the optical phenomenon called asterism, a star-like pattern created on the surface of a gemstone when light encounters parallel fibrous, or needle-like, inclusions within its crystal structure.
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