Short answer: fluorite is calcium fluoride, and its atoms are arranged in a cubic pattern. When a crystal has room to grow, the outside shape follows the inside pattern. That is why fluorite so often forms sharp, natural cubes.
The cubic crystal system
Every mineral has an internal arrangement of atoms that repeats in three dimensions. In fluorite that arrangement has the symmetry of a cube. Crystals grow by adding material layer by layer, so the faces that develop reflect that symmetry.
Cubes are the most familiar result, but not the only one. Fluorite also grows as octahedra, an eight-sided shape like two pyramids joined at the base, and as combinations of the two.
Cleavage: why you see so many octahedra
Fluorite breaks cleanly along four directions. Split a piece along all four and you get an octahedron. Many small fluorite octahedra offered for sale were shaped this way from larger pieces, not grown that way. Both are genuine fluorite. A listing should tell you which one you are looking at.
That same cleavage is a handling warning. A sharp knock can split a crystal along a flat plane.
Color and zoning
Pure fluorite is colorless. Trace elements and small defects in the structure produce purple, green, blue, yellow, and more. Conditions often change while a crystal grows, which leaves bands of different color. Polished rainbow fluorite shows that banding clearly.
The mineral that named fluorescence
In 1852 the physicist George Stokes studied the glow that some fluorite gives off under ultraviolet light. He named the effect fluorescence, after fluorspar, the older name for fluorite.
Not every piece of fluorite fluoresces. The response depends on the trace elements present, and it differs from one locality to the next. If UV response matters to you, look for a listing that documents it for that specific piece.
Handling and care
Fluorite defines 4 on the Mohs hardness scale. A steel blade will scratch it, and so will quartz dust. Keep it apart from harder stones, wipe it with a soft cloth, and display it away from prolonged direct sun, since some colors can fade over time.
What to look for
- Sharp, undamaged edges on natural cubes.
- Whether the shape is natural or cleaved or polished, stated plainly.
- Color zoning that is visible in the photographs.
- Weight and dimensions, since fluorite photographs larger than it is.
Browse fluorite specimens at Digital Towns Market
Digital Towns Market
Fluorite in stock
Sources
- Mindat, fluorite mineral data: https://www.mindat.org/min-1576.html
- Natural History Museum of Utah, What Is Fluorite?: https://nhmu.utah.edu/articles/what-is-fluorite
- GIA, Fluorescence and Phosphorescence (2024): https://www.gia.edu/gems-gemology/winter-2024-fluorescence-phosphorescence