Fluorite

Fluorite, calcium fluoride or CaF₂, belongs to the halide mineral group. Fluorite isn't the most common halide—common salt or halite takes that title—but you'll find it in every rockhound's collection. Fluorite (be careful not to spell it "flourite") forms at shallow depths and relatively cool conditions where deep fluorine-bearing fluids, like the last juices of plutonic intrusions or the strong brines that deposit ores, invade sedimentary rocks with lots of calcium, like limestone.

Mineral collectors prize fluorite for its very wide range of colors, but it's best known for purple. It also often shows different fluorescent colors under ultraviolet light. And some fluorite specimens display thermoluminescence, emitting light as they are heated. No other mineral displays so many kinds of visual interest. Fluorite also occurs in several different crystal forms.

Every rockhound keeps a piece of fluorite handy because it's the standard for hardness 4 in the Mohs scale.

This is not a fluorite crystal, but a broken piece. Fluorite breaks cleanly along three different directions, yielding eight-sided stones—that is, it has perfect octahedral cleavage. Usually fluorite crystals are cubic, like halite, but they can also be octahedral and other shapes too. You can get a nice little cleavage fragment like this at any rock shop.

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