The name Chrysoberyl is derived from the Greek words 'Chrysos' & 'Berullos' meaning 'Golden Crystal'. Although the name refers to the golden yellow colour, it also occurs in a lighter yellow and shades of green and brown.
Chatoyant material is cut as a cabochon (en-cabochon) to display a fine cat's eye effect. The term Cat's Eye, without reference to a particular gemstone name, is used to indicate that the gemstone is a chrysoberyl. Other cat's eye gemstones are named, for example, cat's eye quartz, cat's eye tourmaline etc.
Cat's eye is the most precious of all chatoyant gemstones, it has a sharper eye than that of other chatoyant stones. The inclusions that cause the eye effect are very fine oriented needles of rutile or other minerals.
Alexandrite is a rare colour change variety of chrysoberyl, follow the link for details.
Chrysoberyl is not normally treated, but cat's eye is often imitated by cat's eye quartz and other chatoyant materials.