Garnet

The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning "dark red", or the Latin word granatus ("grain"), possibly a reference to the Punica granatum ("pomegranate"), a plant with red seeds similar in shape, size, and colour to some garnet crystals.

Six common species of garnet are recognized by their chemical composition. They are pyrope, almandine, spessartine, grossular (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite.

Color Change Garnets
Garnet members of the pyrope-spessartine solid-solution series from Bekily in Madagascar display several colours depending on the light source. The alexandrite-like colour change from blue-green in daylight to purple in incandescent light is mainly caused by relatively high amounts of vanadium. Although they look a lot like alexandrites they are different because they change colour throughout the day. They are green or blue grey in the early morning and reddish in the late afternoon or in strong sunlight. Bekily garnets will appear red in the afternoon while the alexandrites remain green. Garnets from other parts of East Africa also change colour but as they normally change from brown or orange to red, they don't look much like alexandrite.

Some of the stones are almost blue especially under fluorescent light but most of them are grey blue or green in daylight and change to red under incandescent or late afternoon light. The stones can show an excellent colour change and can easily be confused with alexandrite.

Source

 * http://en.wikipedia.org