This kind of thing isn't that uncommon. Sapphire and Ruby are the same stone. Iron or titanium impurities turn it blue, or chromium turns it red. Chromium is highly fluorescent. As to the wavelength of red, it's identical to a ruby laser. (too lazy to look up that wavelength atm). Now if a stone has both red and blue color causing impurities, but the red is weak, and only shows up with a little fluorescence, then the color shifts according to the amount of UV light present. There are quite a wide variety of color changing stones. Alexandrite for instance is characterized by a very strong visible color shift from purple-red to green, yet shines a nice red under 405nm light. Granted the only alexandrites I have tested a blu-ray on are cat's eye alexandrites, but I assume the same goes for the faceted transparent variety.
As a side note, most fake alexandrite is sapphire based, as it's much easier to synthesize lab grown corundum (sapphire) with that color shift then chrysoberyl.