Alexanderite 

 

 Ordinary chrysoberyl is yellowish-green and transparent to translucent. 

 

 

When the mineral exhibits good pale green to yellow color and is transparent, then it is used as a gemstone. 

 

The three main varieties of chrysoberyl are: ordinary yellow-to-green chrysoberyl, cat's eye or cymophane, and alexandrite.      (thanx to wikipedia)