30.05.2022 Star Sapphire & Star Ruby
Star sapphires and star rubies show the eponymous star-shaped shimmer that seems to travel across the surface when you move the gemstone. Sapphires and rubies belong to the corundum family and are the second hardest minerals in the world after diamonds, with a Mohs hardness of 9.
How star sapphires and star rubies develop
Corundum occurs frequently, but colored stones in clear gem quality are rare. In its pure form corundum is colorless, the admixture of other elements is responsible for the color. Red corundum is called ruby, all other color varieties (white, yellow, orange, pink, blue) are called sapphire. The most important finding places for sapphires and rubies are in Asia and Africa, for star sapphires for example in Sri Lanka. Corundum is mainly formed in magmatic rocks like pegmatite or garnet but is also found in marble and gneiss.
The star phenomenon, also called asterism, is caused by inclusions of rutile in corundum. Inclusions are very common in rubies, especially inclusions of the titanium oxide rutile. The presence of rutile needles is desirable because if the crystal needles are intact, it is an indication that the gemstone is untreated. High temperature treatments of 1200 degrees and more can remove rutile from rubies and sapphires as it begins to melt.
The cut brings out the perfect star
Asterism occurs only when a sapphire or ruby is cut as a cabochon. Ideally, the stone is cut so that the intersection of the rutile inclusions is approximately at the center of the stone. The star-shaped radiance can be seen in both artificial and natural light. The most common are 6-rayed stars, but there are also 4-, 12-, 18- and 24-rayed ones.
The largest star sapphire ever cut, weighing 536 carats, is the so-called “Star of India”, now on display at the Museum of Natural History in New York. However, the “Star of Adam”, named after its place of discovery – Adams Peak in Sri Lanka – , is the largest star sapphire found so far with a weight of 1404 carats.
Star sapphires and star rubies as investment gemstones
Inclusions often reduce the value of gemstones, but the play of color that an asterism creates contributes positively to the price. However, prices for star sapphires and rubies are lower than those of pure sapphires and rubies without inclusions. As with all colored gemstones, natural-colored, untreated stones with an international certificate are interesting as investments; for star sapphires and star rubies, gemstones from a size of 1 ct. and more are valuable investment gemstones.