Rainbow Moonstone has a monoclinic crystal system and is in the mineral class of lattice silicate in the feldspar family. While Moonstone in general occurs in several varying shades and colors, the Rainbow Moonstone is generally white, to almost clear at times, and usually has a bluish shimmer to it. This color effect is due to its inner structure being a laminate of feldspar which bends light, the same effect as the flash that comes from Labradorite. In this one though, it is potassium that provides the shimmering blue color effect.
While I can not say it has been scientifically proven, I had read a long time ago that its coloring would deepen and become more brilliant when the moon was full. I have watched many of my pieces over the years and I have to say that some, but not all, definitely seem to have a much deeper blue coloring at the time of the full moon, and they also seem to acquire increased energies as well when this occurs. My daughter has also noticed this in one ring that she wears, and others have mentioned observing the same effect in pieces that they have acquired.