Star Sapphire
Some specimens of sapphire contain a very fine "silk" of fibrous inclusions that parallel the crystallographic axes of the mineral. When these stones are cut into a cabochon with the c-axis of the stone penetrating the base of the stone at right angles, a six-rayed star can be seen floating on the surface of the cabochon.
These are known as "star sapphires".
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Star sapphires belong to the phenomenal corundum category.
Corundum is an exceptionally hard and tough material. It is the third-hardest mineral, after diamond and moissanite. It serves as the index mineral for a hardness of nine on the Mohs Hardness Scale.
The fine "silk" fibrous inclusions inside star sapphires.
The star effect in star sapphires is called asterism. It’s caused by reflections from tiny, needle-like inclusions that are oriented in several specific directions. Stars are usually made up of 2, 3, or 6 intersecting bands, resulting in 4, 6, or 12 rays.
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The finest star is distinct, centered on the top of the stone, and visible from a reasonable distance, about arm’s length. The star’s quality should be the same when viewed from all directions.
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The best and most expensive star corundum is semi-transparent, with just enough silk to create a well-defined star. Too much silk can harm transparency and also lead to poor color, lowering the value of the stone considerably.
Star corundum must be cut as a cabochon to display asterism. A finished stone’s attractiveness depends on the star’s orientation and the cabochon’s symmetry, proportions, and finish.
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The cabochon must have an appealing appearance, with the star properly centered when the gem rests on its base. The stone’s outline should be symmetrical.
Noteworthy locations where gem-quality corundums have been produced include Myanmar, Thailand, Cambodia, Vietnam, India, Pakistan, Afghanistan, Sri Lanka, China, Australia, Madagascar, Kenya, Tanzania, Nigeria, and Malawi.
Trivia: Sapphire is a birthstone for September. It is also the gem for a 5th anniversary.