Although many people have never heard of chrysoberyl, there are two gems that belong to this group that are very valuable and unique. They are alexandrite and chrysoberyl cat's-eye; the first is usually sold in faceted form and the other is always sold as a cabochon (a domed shape, much like lapis, opal and other opaque or translucent gems).  Both of these gems exhibit what gemologists call phenomena; an unusual trait that sets it apart from most other gems. In Alexandrite's case, it has the ability to go from a red color to a green color depending on the light source. This is because  the cellular structure of alexandrite absorbs and reflects certain wavelengths of light and this causes our eyes to see either red or green when viewing the gem. It is quite entertaining to see how a fine alexandrite will go from an emerald green color in daylight to a ruby red color in incandescent light. The phrase "Emerald by day; Ruby by night" can help you to remember the color change. Alexandrite that exhibits this type of color change and in relatively pure colors of green and red, command very high prices.  It is not uncommon to find this type of material priced from $15,000 to $24,000/ct. retail.  Also it is very rare to find this type of quality in gems over one carat in size.  So, it is obvious we are talking about a very rare gem with an outstanding phenomenon, very unique in the gem world.

Having said that, please be aware that the alexandrite that tourists bring back from Mexico and other places, usually set in silver, is not natural alexandrite.  It is a synthetic product that has a color change, usually from a raspberry or purplish color to a greenish blue color.  Also, it can be found in large sizes.  It is not valuable and can be worn and enjoyed for what it is, costume or fashion jewelry.

Alexandrite is one gem that it is absolutely critical that you buy from a trusted source since the price can vary so widely based on such factors as the purity and type of color and how distinct is the color change.  This is a gem that is very expensive in its finest form and isn't worth near as much if the color change is weak or muddy.  Deal only with a gem professional, preferably a graduate gemologist, when contemplating such a purchase. 

Chrysoberyl cat's-eye is another gem with an unusual characteristic.  When light is aimed at the gem, it will exhibit a light colored narrow band that runs the length of the gem.  When two light sources are used, one stationary directly above the gem, and the other moving along the side to the girdle, the band will "open" like a cat's eye.  When the light returns from the girdle area back to the top where the other stationary light is, the "eye" closes.  So the eye will open and close depending on where the light source is.  This phenomenon is due to fine, parallel inclusions within the gem material that are highly reflective.  Often, it is said to show a milk and honey effect.  This pertains to the color variations noted when the eye opens and closes.  Fine chrysoberyl cat's-eye material  can sell for $2,000/ct to $3,000/ct retail.  This gem is excellent for a man's ring since it is very durable, distinctive and with subtle colors.