I have been fortunate enough to have designed several pieces with different types of sapphires. The faceted sapphires are well known and extremely popular but the star sapphires are not as commonly found or used. So what exactly is the “star” in star sapphires and what causes this incredible natural phenomenon?
Most people don’t realize that sapphires and rubies come from the same rock-forming mineral called corundum. It is merely the metal content within the corundum that gives the gem its unique color which also has the greatest influence on the value of the sapphire.
What is Asterism?
The “star” phenomenon in a sapphire is called asterism and is caused by reflections from tiny, needle-like inclusions clustered together. These clusters are oriented in several specific directions which make up the star. The bands intersect in the center and result in a dazzling display of light with the rays dispersing in four, five, six, or twelve (very rare). Star sapphires are traditionally cut as cabochons because the curved dome of the gemstone focuses the light like a spotlight.
How do you choose a star sapphire?
If you are looking to choose a star sapphire here are a few key points to look for.
The finest star is distinct and centered on the top of the gemstone and should be visible from a reasonable distance (about arm’s length).
The rays should be uniform in strength and evenly distributed when viewed from all directions.
The rays should also reach from girdle (side of the gemstone where the prongs are set) to girdle and should be straight, distinct, and not fuzzy.
They should also be elongated and have “movement” meaning as you rock the gemstone the star “moves” across the surface.
The best and most expensive star sapphire is semi-transparent with just the right smattering of “silk” or inclusions to create a well-defined star.
Star sapphires are so unique in shape and size that they usually demand a custom setting. They can also be difficult to set because of their rounded shape and need a skilled goldsmith to masterfully handle them. Having a less experienced designer or goldsmith working with this gemstone can result in breakage.
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