"Blue Halite" - Phenomenal Optical Illusion of Blue
What do you see? Blue, blue, blue! That's the question people often ask when they are looking at blue halite. To some, this particular halite appears to be evidently blue, but to others it looks colorless—but what's the true answer? That depends on how the lattice disorder of the crystal interacts with light. If you bend and reflect light off this crystalline structure (blue halite), you will create the illusion of blue color - because of the chemical reaction that creates the lattice disorder within the crystalline structure!
Blue halite forms when a solution of potassium chloride and sodium sulfate evaporates. The potassium reacts with the sodium to cause a blue coloration in the crystal lattice structure (a reaction between two non-metals). If there were no disorder within this crystalline structure, then you would see a clear or white crystal; however, the disorder caused by this evaporated solution creates an optical illusion that makes it look blue.
This type of natural salt is only found in two locations in the entire world: Poland and Mexico.