Star Stones (Asterism)  Stones falling within this group include Star Stones (this phenomenon is commonly referred to as Asterism) for example
a Star Ruby or Star Sapphire. These stones are cut as "en cabochon" to fully show off the star effect, it
takes a skilled stone cutter to bring out the best the stone has to offer. The star may have as few as 4 rays or up
to as many as 12 rays radiating outward from the central point
The
asterism effect is caused by tiny needle like inclusions that reflect and scatter light within the stone. One of the largest and finest quality star rubies
on public display is housed at the Smithsonian Institute - the 137 ct Rosser-Reeves Ruby. [Photo courtesy
of ScienceViews.com SIA3098: Rosser Reeves Star Ruby]  
Cat's-Eye (Chatoyancy)  While a chatoyant gemstone looks similar to the star stones, it should have just one line of varying thickness - hence the
term 'cat's-eye' effect. This can occur in different types of gemstone, however, the term "cat's-eye" always
refers to the mineral Chrysoberyl. If it is used with any other gemstone it must be designated as such i.e. cat's-eye
quartz or cat's-eye tourmaline and so on.
Again, in order to display the
effect of the stone at is best, they are cut "en cabochon" as with the star stones. [Photo courtesy
of ScienceViews.com SIA3103: Maharani Cat's-Eye]
Color Change Gemstones Color-Change also referred to as Photochroism is not commonly found in many gemstones. Probably the most
well known for this effect is Alexandrite which is a member of the Chrysoberyl species. Which when exposed to daylight
and then incandescent light can change color, the most prized being bluish green to a reddish color, and the more intense
the color change, the more valuable the stone. This
effect can also be found in garnet and sapphire which are less costly than Alexandrite and some of the more exotic stones. Color-change gemstones are much prized by gem collectors, and
designers alike. They truly are an amazing feat of nature.
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