Fluorite

Fluorite is a halide mineral, calcium fluoride (CaF₂), and the natural source of the element fluorine. Fluorite crystallizes from hydrothermal fluids - hot, mineral-rich water circulating through fractures in the Earth's crust. As these fluids cool and the dissolved calcium fluoride reaches saturation, fluorite begins to precipitate, atom by atom, building its characteristic cubic crystal structure.

What makes fluorite remarkable is its staggering range of colors. The pure mineral is actually colorless - every hue you see is caused by defects in the crystal lattice. When radiation from nearby radioactive minerals knocks electrons out of position, these displaced electrons create 'color centers' that absorb specific wavelengths of light. Different defect configurations produce different colors, which is why a single fluorite crystal can display multiple bands of purple, green, blue, and yellow.

Fluorite also gives its name to fluorescence - the phenomenon was first scientifically described in fluorite specimens. Many fluorite crystals glow vivid blue or purple under ultraviolet light, caused by trace amounts of europium or other rare earth elements substituting for calcium in the crystal structure.

Fluorite's perfect octahedral cleavage is its most distinctive diagnostic feature. It breaks cleanly along four planes, producing fragments shaped like octahedrons (eight-faced solids). No other common mineral cleaves this way. At hardness 4, fluorite can be scratched by a steel knife but not by a copper coin.

Its cubic crystal habit is also diagnostic. Fluorite typically forms as cubes, octahedrons, or combinations of both. The wide color range can cause confusion with amethyst (purple varieties), emerald (green), and even sapphire (blue), but the hardness test immediately resolves these: fluorite at 4 is dramatically softer than all of them. Testing with a UV flashlight can also help, since many fluorite specimens fluoresce strongly.