Fluorite.

Fluorite is calcium fluoride, a halide that crystallises in clean cubes and octahedra at almost every scale from millimetres to handspan. Trace impurities and natural irradiation produce the wide colour range: yttrium for purple, samarium for green, tin for blue. Many specimens show colour banding within a single crystal, and the rainbow fluorite of the Chinese deposits stacks several hues in a single polished slab.

The mineral gave its name to fluorescence as a phenomenon. Nineteenth-century chemists noticed that some fluorite samples glowed under ultraviolet light, and the term entered scientific vocabulary from the mineral itself. The name fluorite traces further back to the Latin fluere, to flow, because the stone was used as a flux in iron smelting to lower the melting point of slag.

In modern crystal practice fluorite is closely associated with focused study and mental ordering. A piece on the desk is the most common placement, and the colour you choose tends to follow the kind of work: green for steady learning, purple for more intuitive or reflective work, blue for clear speech and writing. The stone is soft enough to scratch with a steel knife, so it suits a shelf piece better than a pocket carry. Keep it away from prolonged sun, which can fade some varieties, and clean only with a soft cloth.