A star ruby, star sapphire, diamond and pearl cross by Luis Miguel Howard. Note the difference in colour in the star rubies and sapphires.

I have always been fascinated by and drawn to more unusual stones; star rubies and sapphires have an ethereal beauty to them and I have always considered them rather underrated. Their allure is not immediately obvious as the star is not immediately obvious if viewed in natural daylight. But shine a light on it and a crisp, white, iridescent star will appear to reveal itself from within the depths of the stone. For me, it was love at first sight.

In this gemstone, it is a case of perfection born from imperfections. Rubies and sapphires are the same stone- the corundum family. The only difference between them is that when the presence of chromium is strong enough it to be red and therefore classes as a ruby. Any other coloured corundum is a coloured sapphire. Most rubies and sapphires contain naturally occurring flaws called rutiles- microscopic, needle like flaws which are known as silk. A star is produced when the stone contains an abundance of silk, all criss-crossing within the stone which causes the light to diffuse and produce a six ray star effect. This effect in gemmology is known as asterism.

A top quality star gem should be a rich, saturated colour with little or no secondary hues or overtones. High transparency, so highly prized in conventional stones, will only result in a poor star, so good star rubies and sapphires will always be semi opaque. The star should be crisp and almost luminous, with the rays being perfectly straight. Star rubies and sapphires are always cut cabochon, the only way to release the optical effect within. The star should also always be visible when exposed to any kind of artificial light- if you have to hold the stone this way and that to appreciate it, it is not good enough. The stone should also be clean- there should be no black spots or any other visible inclusions of any kind. With no criss-crossing facets to hide inclusions under, clean stones are rarer and therefore more expensive.

The highest quality gems come from Burma for rubies and Sri Lanka for sapphires. That said, the largest star ruby in the world, the 138 carat Rosser Reeves in the Smithsonian is Sri Lankan. Demand for perfect rubies and sapphires has soared (especially in Asia) and fewer gems of larger sizes (five carats and above) are being mined. This has put enormous pressure on wholesalers to satisfy the market- most star rubies and sapphires of exceptional quality will be heat treated (this is not uncommon: 90% of the world’s coloured gems are heat treated) to dissolve the silk and produce the coveted faceted gemstones which are achieving world record prices at the moment. As such, perfect star rubies and sapphires are extremely rare- especially rubies (which are already the world’s rarest gemstone), where you can expect to pay $50000 a carat for a perfect example. A perfect blue is not so rare, but you can still expect to part with over $5000 a carat if the stone is over 5 carats.

However, if on a budget (as most of us are) one can easily compromise on the colour- and sometimes I find that ‘less than perfect’ stones will suit a jewel or a wearer almost better than the textbook example. There are beautiful examples to be found in greys, soft blues and pink for a tenth of the price quoted above- if you are thinking of buying one, just don’t compromise on the star- after all, that is the point of a star gem. Above all, choose a stone that speaks to you.