Purple Fluorite

Fluorite is calcium fluoride, and it is unusually important to science for a soft ornamental mineral. It gave its name to two phenomena: fluorescence, the glow of certain fluorite specimens under ultraviolet light, named after the stone by George Stokes in 1852; and the element fluorine, first isolated from it. It is the reference point for hardness 4 on the Mohs scale, has perfect octahedral cleavage that lets it be split into clean eight-sided pieces, and occurs in nearly every colour, purple being among the most prized. As a flux in smelting, the Latin fluere, to flow, is the root of the name, it has a long industrial history, and the banded blue-purple Blue John of Derbyshire was carved into Georgian ornaments. Its metaphysical placement, typically with mental clarity and the higher centres, is a modern colour-and-name-led assignment, not a traditional one.

Fluorite has long been carved in China as an inexpensive stand-in for jade and other harder stones; called yingshi in Chinese lapidary tradition, banded and coloured fluorite was cut into snuff bottles, small vessels and figures from at least the Qing dynasty onward, prized for taking a high polish despite its softness. In Europe the same softness was turned to different use in Derbyshire, England, where a rare banded purple-and-yellow fluorite called Blue John was mined from a single hill, Treak Cliff, and worked into vases and ornaments from the eighteenth century, a British industry now nearly exhausted of its raw material.

Fluorite has given two words to the language and both came from watching it behave.

The first is flux. Fluorite is calcium fluoride, it melts at a relatively low temperature, and it was added to metal ores to make the slag run freely. The name comes from Latin fluere, to flow, and the mineral is still used in enormous quantities in steelmaking for exactly that purpose, as well as being the principal source of fluorine and of hydrofluoric acid.

The second is fluorescence. Many fluorites glow, typically blue or violet, under ultraviolet light. George Gabriel Stokes described the effect in 1852 and named it after the mineral in which he had been observing it, so every fluorescent lamp and every fluorescent dye is named, at one remove, after this stone. Not all fluorite fluoresces, and the ones that do owe it to trace rare earth elements rather than to the calcium fluoride itself.

The purple banded variety from Castleton in Derbyshire, Blue John, was worked into vases and urns in the eighteenth and nineteenth centuries and is found in only that one small area. It is sometimes proposed as the material of the Roman murrina vessels that Pliny describes as extravagantly costly, but the identification is disputed and the ancient descriptions do not settle it.

Fluorite is soft, four on the Mohs scale, and it has perfect cleavage in four directions, which means it can be cleaved into octahedra and that it is a poor material for anything that will be worn.