Crystal
The stone that gave fluorescence its name — banded purple and green, and soft enough to be handled carefully.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Calcium fluoride sits at 4 on the Mohs scale because Mohs put it there; it is one of the ten defining minerals of the scale. It grows in cubes and octahedra, in almost every colour, and frequently in several colours at once as banded zones. It also cleaves perfectly in four directions, which is why the octahedra sold in shops are usually cleaved rather than grown that way, and why fluorite chips if you drop it.
Two words come from it. Fluorite was used as a flux in smelting — from Latin fluere, to flow — and when George Gabriel Stokes described the blue glow certain minerals give under ultraviolet light in 1852, he named the effect fluorescence after this stone. Blue John, the banded purple-and-yellow variety from Castleton in Derbyshire, has been mined in the same two small caverns for three hundred years and is found essentially nowhere else on Earth.
Practice files it under concentration and mental order, and it is one of the correspondences that has stayed remarkably stable since it entered the literature. A practical note: fluorite is soft and its colours can fade in strong sunlight, so it is not a stone for a car dashboard or a windowsill.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Common in contemporary practice
Kept beside work that requires sustained attention, particularly the banded pieces, on the reading that the visible zones stand for orderly separation of one thing from another. It is a frankly literary correspondence and practitioners know it.
Carried forward
Common in contemporary practice
Green and purple fluorite are often assigned different roles — green to the practical, purple to the reflective — though the colour difference is a matter of trace elements and irradiation and nothing else. Nobody has offered an account of why that should matter.
Credit what you carry. Much of this comes from living traditions, and some of it from traditions that are closed — held by particular peoples and not offered for general use. Where you know whose practice a reading came from, name them; where you don't, say that too.