Asterism – Explaining the Star Sapphire and Star Ruby
Star sapphires and star rubies are the rarest and most valuable stones of their type. The shifting star pattern which shines within them has an otherworldly quality – perfectly symmetrical and shifting in the light. It is no surprise that they command the highest prices and are set into the most exquisite of jewellery, but what causes a stone to develop a “star” within it?
Star stones develop their impressive pattern due to a phenomenon called asterism, a word derived from the ancient Greek word for star. When the stone is formed it includes uniform impurities of titanium dioxide contained within the mineral rutile. These tiny filaments, colloquially known as “silk”, are laced throughout the entire stone, giving star stones a milky, translucent colouring.
When light shines on a star stone the stone’s base material and the rutile impurities refract light at a different index. It is this dichotomy that causes light to form into thin, needle like points which coalesce at the stones heart.
The star effect is not static. It is formed by the movement of light, and shifts when the stone is viewed from a different angle. Opals exhibit a similar phenomenon known as opalescence, where ribbons of silica within the stone reflect different wavelengths of light, causing the colours within to shift between radical extremes.
Star patterns form most readily in corundum, the mineral from which sapphires and rubies are formed. Although all rubies are red they vary in colour from the deep crimson of blood to an almost neon bright pink. Sapphires can be even more varied. The most recognisable are a deep blue, but yellow, green and pink sapphires also exist.
Star patterns can form in all colours of corundum, but they can also form in other gemstones. These formations are rare, but star impurities have been documented in garnet, diopside and spinel.
Whilst rutile inclusions are responsible for the bright white star patterns which are most readily recognisable it is also possible for a stone to take on hematite impurities, which produce a darker star pattern. It is even possible for a stone to contain both rutile and hematite, and to exhibit both stars at once, creating a twelve pointed star rather than the standard six.
Such stones are the rarest of the rare, and their value borders on the priceless.
At Antique Jewellery Online we are constantly updating our inventory with beautiful pieces we have discovered. With over 50 years of experience in the trade we have handled tens of thousands of pieces, and yet we have acquired only a handful of star stone pieces.
You can view our selection of star sapphires here and star rubies here.