The gem phenomena of asterism doesn’t come along everyday—here’s everything you need to know about rare and precious Star Sapphires.
Phenomenal gems are those that feature very special optical effects, such as, Opal’s play-of-colour, Cat’s Eye Chrysoberyl, Moonstone, colour-change gems such as Alexandrite, and of course, Star Rubies and Sapphires. Similar to cat’s eye gems, the effect of asterism is actually caused by a rare collection of inclusions. Inclusions are often considered a negative effect, as they often take away from a stone's natural perfection. However, in the case of cat’s eye and asterism effects, they form in a such a rare and unique way that they create a very special collector’s piece.
Asterism is created when long, thin, tube-like inclusions (often called rutile inclusions) form in a large group. This group often creates the effect of “silk” in a gem, since the inclusions diffuse light and often give the surface of the gemstone a lovely silky appearance. However, when the entire gem features these tube-like inclusions and they are highly uniform and parallel, they can reflect a straight and sharp line of light, similar to how a spool of thread does. Singularly, this creates the cat’s eye effect, however when there are multiple groups of rutile inclusions, each formed in parallel to each side of a Ruby or Sapphire’s six crystal sides, multiple distinct lines of light can appear and form a six-pointed star. In very rare conditions, multiple sets of these inclusions can even form a 12-pointed star. Stars can appear in any Sapphire colour, including blue, grey, black, purple, yellow, and red (which are Star Rubies), and must be cut into a cabochon shape in order for the asterism effect to be seen. Asterism can sometimes also be seen in Garnet, Spinel and Diopside; although they are significantly less popular.
Star Rubies and Sapphires have been documented since the 1500s, and treasured as special talismans ever since. Due to their otherworldly symbolism, is has often been thought that they could help one identify their true purpose, while also helping with meditation and mental focus, and they have often been referred to as “stones of destiny.” In the 1700s, what is still the world’s largest blue Star Sapphire was found. Weighing over 563 carats, the Star of India was unearthed in Sri Lanka, and now resides in NYC’s American Museum of Natural History. Since then, Sri Lanka has remained one of the primary sources of Star Sapphires, however they can naturally be found wherever standard Sapphires are mined, including Myanmar, Australia, Thailand, Vietnam, Africa and the United States.
The best quality Star Sapphires have saturated, semi-transparent body colour, with a well-defined, symmetrical star that is centred on the top of the gem, with rays which extend uniformly to the edges, or girdles, of the stone. As you tilt the cabochon in direct light, the star should float across the surface of the cabochon evenly and without any dead spots. It’s best to view the effect under spot lighting, as they naturally won’t show as well in diffused and fluorescent light, including sunlight. Just like standard Sapphires, Rubies are the most rare and valued asterism gems, while black and grey are the least sought-after.
While certainly an enchanting gemstone of intrigue and allure, Star gems are much more than just a novelty, and can certainly become the luminary of any jewellery piece or collection.