(Silk Inclusions in Sapphire; image courtesy of GIA edu)
Where clarity is concerned, inclusions occur in sapphires too but not to the extent that they occur in rubies. Some of these inclusions are needle thin and intersect each other are known as silk, while others can form colour bands or the shape similar to fingerprints.
(Pleochroism; image courtesy of GIA edu)
These inclusions can cause the price of the gem to drop or soar depending on the effect they have on its clarity. The star effect, for instance, is called asterism and can hike up the price of the stone if the star is well-defined. Pleochroism is the phenomenon that occurs when the faceted stone shows off a different colour when looked at from a different direction. This is also quite an important characteristic of a sapphire.
(Star Sapphire set in a ring; image courtesy of Sotheby’s)
Now, colour zoning refers to the areas of different colours in a stone which is characteristic of a sapphire. Cutters often cut the gem in accordance with a pattern that shows the colour in the best possible perspective. The star corundums are, of course, carved in a cabochon cut (not faceted) with the star visible in its centre, the rays equidistant from each other. The value of a star corundum is decided on the basis of the carats (weight), the colour, the visibility and intensity of the asterism (star effect).
The weight of these sapphires can range from a few to a hundred carats. The price can change according to the size of the gemstone depending on the quality of the stone which can be fine-quality or commercial quality (far more easily available).
(Clockwise: The Star of Adam, the Star of Bombay and the Star of India; images courtesy of Dailymail UK, Flickr and Smithsonian Mag.com respectively.)
The largest (famed) sapphire is the Star of Adam weighs 1404.49 carats, the Star of India weighs 563.4 carats and the Star of Bombay 182 carats and all of these stones were mined in Sri Lanka but more about these in the next post.