By Rachael Taylor
Gaze into the deep blue - or perhaps lilac or grey - of a star sapphire cabochon and you'll be greeted with an optical phenomenon known as asterism. When the light hits the stone, a crisp criss-cross of white dances across the smooth surface, creating a naturally occurring star of light.
As with all gemstones, there are key factors that make the difference between one you can pick up in a hobbyists' shop and the crème de la crème that sell for thousands. For star sapphires, it is the colour - dark, rich blues command the highest prices - and also the visibility of the star, which should be bright, clear and centrally aligned.
Auctions are a good place to seek out star sapphires, both loose and set into antique jewellery. In November last year Bonhams sold a rich cornflower blue 24.36ct Sri Lankan star sapphire for $27,500, while the year before a Van Cleef & Arpels star sapphire and diamond brooch tripled its pre-sale estimate when it sold at Christie's for $32,000.
When it comes to contemporary jewellery, tracking down a star sapphire of a quality that means you don't have to carry a torch with you to show off its defining feature is, quite frankly, a tall order. These gems are rare and expensive, and as such not many jewellers are working with them.
But they are out there. Jewellers hunting down quality star sapphires include Oscar Heyman in New York, and Robinson Pelham and Ben Day in London. DeLaneau has also created a positive feast of asterism with its Grace Stars jewellery watch that uses not just star sapphires but also star rubies.
Ben Day, who has spent the past three decades specialising in rare coloured gemstones, describes star sapphires as "magical and otherworldly". In his workshop right now is a "particularly beautiful" 7.72ct unheated star sapphire from Sri Lanka, set in an 18ct rose gold ring with pink sapphire pavé.
"Just the soft blue of the sapphire is beautiful, subtle and refined, but when the light hits and this pure crisp white star is revealed it takes on a whole new life," says Day. "This transformation always stops me in my tracks."
And so you have it. The real allure of a star sapphire is not its price or its scarcity, but the magic of that moment when a meditative gaze into the deep blue is electrified by an energising burst of light.