Sunlight 
At midday, it normally has a neutral effect on the hue. Earlier and later in the day, it adds red, orange, or yellow making the
stones look more purple.

•Light bulbs & candlelight 
Add red. Purple colors are strengthened, blue may turn violet to purple, and grayish colors may look brownish. The degrees of change varies depending on the stone. Some show little change.

•Flourescent lights
Depends on what type they are. Most strengthen the blue in
tanzanite.

•Halogen spotlights
Add sparkle and usually make stones look more purple. The
color change is usually less than with light bulbs.

•Light under an overcast sky 
Adds blue and grey.