Fluorite

Fluorite is the crystalized form of Calcium Fluoride.  It crystallizes in cubes.  Next to Quartz, Fluorite is the world's most collected mineral.  Pure Fluorite is colorless.  Its color comes from whatever was present at the time it crystalized.  Depending on the impurity creating the color, any color is possible, but green, purple, blue and yellow are the most common. 

Fluorite meaning: Its name comes from a Latin words that means to flow.  It got this name because it easily melts.

Fluorite forms in cubes and there can be cubes inside of cubes.  Sometimes there is a phantom cube inside another cube.  You can see a cube of one color inside a clear cube or another cube of color.

Fluorite forms octahedrons because it cleaves perfectly in all four directions.