Star sapphire

Star sapphire is sapphire that shows a six-rayed star of light on its surface, thanks to extremely fine needle-like inclusions inside it. The effect is called asterism. The stone is cut as a dome (cabochon) to bring out the star. It can be blue, greyish blue, purple or black. Most of the world's famous star sapphires come from Sri Lanka, while black star sapphires come mainly from Thailand and Australia.

Jewellery: Star sapphire is set as a dome-cut stone in rings, necklaces and earrings; its hardness suits everyday wear. Collections and museums: Giant stones such as the Star of India, the Star of Bombay and the Star of Asia are on display in museums in New York and Washington. Talisman and ornament: It has been carried as a talisman through history; in 1913 the Greeks of America gave Prince Constantine a sword with a large star sapphire in its hilt.

It has a hardness of 9 on the Mohs scale. It scratches glass and steel; a steel knife cannot scratch it. Its specific gravity is 3.98–4.1 g/cm³.

Its best-known localities: Ratnapura, Sabaragamuwa, Sri Lanka; Bang Kha Cha, Chanthaburi, Thailand; Pailin, Cambodia; Mogok, Mandalay Region, Myanmar; Central Queensland gemfields (Rubyvale, Sapphire, Anakie), Australia.

A dome-cut stone shows a six-rayed star; as the stone or the light source moves, the star glides across the surface. It is sharpest under a single point of light (sunlight, a torch). Being a sapphire, it is very hard (Mohs 9) and heavy for its size. In natural stones the star is often slightly hazy, the body colour milky or greyish, and colour bands may be visible. A very sharp, flawless star with very even colour can point to a synthetic. Synthetic star sapphire: the method was developed at the Linde company in the United States in 1947; titanium-bearing synthetic corundum is grown first and then heated to form rutile needles inside it. In early production the needles lie only in a layer near the surface.

On the “street to diamond” value ladder it sits on step 7 of nine: Precious stone. In a star sapphire the star itself comes first: the six rays should be sharp, straight, equally long and centred on the stone. Body colour comes next: a saturated blue is the most valuable, while milky, greyish or very dark stones are worth less. The height and symmetry of the dome, size, and whether the stone has been heat-treated also matter. Because synthetic and treated stars are common, a laboratory report is decisive in valuing a stone.

According to the mineralogist George F. Kunz (1913), the people of Sri Lanka believed that the gliding star protected against witchcraft of every kind. Kunz reports that the three bars of the star sapphire were believed to stand for faith, hope and destiny, and that the stone was worn against ill omen and the evil eye; it was said to keep working for its first wearer even after changing hands. Kunz tells how the traveller Sir Richard Burton carried a large star sapphire as a talisman that he believed brought him luck and a good welcome; according to Anselmus de Boot in the 17th century, the Germans called the stone a “victory stone” (Siegstein). There is no scientific evidence that star sapphire protects against witchcraft or the evil eye, or brings luck or victory. The star is a purely physical effect produced by light reflecting from fine rutile needles aligned in three directions inside the stone; the same effect can be made in a laboratory. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.