The best Alexandrite was first discovered in Russia in the 1800's and named after the young Czar Alexander Nicolajevitch II.  It is actually part of the Chrysoberyl family with colors ranging from green, greenish yellow and yellow to brown.  There are two varieties, Alexandrite and Cat's-Eye and they both have unique individual qualities. Alexandrite is rare and valuable especially in larger sizes, which makes it expensive in comparison to garnet or sapphire which can also be found as a color change stone. Since genuine Alexandrite today is considered so rare and expensive, most often what you see in stores will be synthetic lab created stones, especially the larger ones.  It should also be clearly indicated if the stones are natural or created. Color change meaning, that in a good quality stone you should see a significant change of color from that seen in natural daylight or flourescent light as compared to the color you see under incandescent lights such as candle light or a regular lamp. Alexandrite is sometimes referred to as "emerald by day and ruby by night" because of the dramatic color change. Alexandrite has the unique ability to absorb different parts of the color spectrum, the term used for how it does that is referred to as "pleochroism".  Okay so now it starts getting a bit technical so I am not going to explain that in depth but it is to do with the stones bi-fringence. Care and Cleaning Alexandrite is a fairly hard stone at 8.5 on the Moh's Scale, so it is excellent for everyday wear.  It does not have treatments of any kind and for this reason you can use different types of cleaning such as Ultrasonic And Steam Cleaning which are usually safe. Personally I would rather use warm soapy water and dry with a soft cloth. Tweet Specific Gravity 3.71 Refractive Values 1.74 - 1.75 birefringence (doubly refractive gems only) 0.009 Lustre Vitreous