Tanzanite comes in a wondrous range of blues and violets but it is also very sensitive to the light source it is viewed under, so the same stone can appear very different in varying circumstances. Anyone who has owned a Tanzanite will know how much of a “mood changer” this gem type is. During the day, in the glare of the white fluorescent strip lights of the office environment, your pendant glows a deep blue and then at night in the restaurant with the low lighting and predominance of incandescent lighting, your friends comment on how the sparks of red in your Tanzanite are so eye catching.

So why is this so? Tanzanite is a trichroic gemstone. This means that Tanzanite’s chemical structure causes the double refraction of white light as it passes through it.  A good illustration is in looking at a Tanzanite crystal in its rough form, turning it on 3 different axes you can perceive 3 different colors.  One axis will appear blue, another reddish brown and the other a deep violet. 

Same Tanzanite crystal from 3 different angles.   Illustrating Tanzanite's trichroism    

It is this trichroism which is responsible for the way your tanzanite behaves under different lighting conditions. Selective absorption of different color components in the light source cause the stone to appear completely different depending on the light it is being viewed under.

This video does an excellent job in demonstrating trichroism at work in a Tanzanite.

 
In white light, like fluorescent strip bulbs commonly found in offices, the stone absorbs more of the long wavelengths (reds, oranges) and emits shorter wavelengths (blues).  In incandescent (warmer light sources) it absorbs more of the shorter wavelengths (blues) and emits longer ones which is why the pinks and reds are visible like in the photographs below of the same piece taken in different light sources.

In White Light & Incandescent light. Red Flash Effect is Evident Under Incandescent  

So next time you hear the words "Red Flash" or wonder why your Tanzanite changes so much at different times of day, in different rooms of your house or in a restaurant, you know why.