Fluorite: the mineral that gave fluorescence its name

Fluorite sits at 4 on the Mohs scale, a standard reference point on the scale itself, and has perfect cleavage in four directions, which is why broken fluorite often shows flat, sometimes octahedral faces even without any cutting. It comes in an enormous range of colors, sometimes several colors zoned within a single crystal. A genuinely interesting fact, the word fluorescence was coined specifically because of fluorite, since many fluorite specimens glow vividly under ultraviolet light, and the phenomenon was first scientifically described using this mineral before the term was applied more broadly to other glowing materials.