Fluorite

What it actually is

Calcium fluoride, CaF2, cubic, hardness 4, with perfect cleavage in four directions parallel to the faces of an octahedron.

That cleavage is the key practical fact. A fluorite cube can be split along four planes to yield a perfect octahedron, and most of the octahedra on sale were made that way rather than grown that way. It also means a knock on a corner takes a clean triangular chip off.

Fluorite occurs in almost every colour, frequently zoned or banded within a single crystal, and the colours come from a mixture of colour centres, rare earth impurities and natural irradiation. Many of them fade in sunlight.

Fluorescence takes its name from this mineral: George Stokes coined the term in 1852 after the blue glow of fluorite from Weardale under ultraviolet light.

Fluorite is also the industrial source of fluorine and of hydrofluoric acid, and optical-grade material is used in high-end camera and telescope lenses for its low dispersion.

Where it comes from

China dominates world production, both industrial and specimen. Mexico produces fine crystals, and Namibia's Okorusu material is prized.

Britain's contribution is historically important. Weardale in County Durham produced the fluorite that gave fluorescence its name, and Castleton in Derbyshire produced Blue John, a banded purple and cream variety mined since the eighteenth century for vases, bowls and inlay. Very little Blue John is now extracted, and antique pieces are valuable.

Roman writers describe murrine vessels made of a banded material commanding extraordinary prices, and Blue John or a similar fluorite is the leading identification.

Illinois in the United States produced the classic Cave-in-Rock material and named fluorite its state mineral.

What the traditions claimed

Fluorite has little ancient lore under that name, since it was not identified as a distinct mineral until the eighteenth century.

The Roman murrine story is the substantial historical thread. Pliny describes vessels of murrina fetching prices comparable to the finest gold work, and records Nero paying an enormous sum for a single cup. If those vessels were fluorite, they represent the highest value ever placed on the material.

Blue John's later history is a British craft tradition, worked in Derbyshire from the eighteenth century into vases, urns and jewellery, some of it exported and much of it now in museums.

Modern crystal practice reads fluorite as a stone of mental order, concentration and study, often recommended for desks and for examinations. That reading is twentieth-century and derives largely from the mineral's geometric perfection and clarity.

It is one of the better modern assignments in the sense that it is at least about what the mineral looks like it is doing.

How it is worked with

The study reading is usable and fluorite makes a good desk object: it is visually orderly, often banded in clean zones, and cheap enough to buy a good-sized piece.

An octahedron is the classic form and is worth owning, though it is worth knowing that most were cleaved rather than grown.

Care is where people go wrong. At hardness 4 with four cleavage directions, fluorite chips if dropped, scratches against quartz dust, and many colours fade in sunlight. Keep it in a lined box or on a shelf away from the window, and never put it in an ultrasonic cleaner.

Water is tolerable briefly but not advisable, since water in a cleavage crack plus a temperature change can split a piece.

For jewellery, fluorite needs a protective setting and gentle treatment, and it is not a daily ring stone.

Buying it honestly

Fluorite is dyed and heat-treated to enhance colour, and coated material exists.

Natural fluorite runs purple, green, blue, yellow, colourless and, in Blue John, banded purple and cream. Extremely saturated single-colour material at low prices is often treated.

Some material is irradiated to deepen or change colour, and irradiated colour can fade quickly.

Glass is sold as fluorite in cheap carvings, and the cleavage test distinguishes them: fluorite chips along flat triangular planes, glass shows conchoidal fracture.

Blue John is valuable enough to imitate, and dyed or resin-cast pieces exist; genuine antique Blue John has a specific banding and is usually stabilised with resin because the natural material is fractured.

Colour-zoned specimens are sometimes assembled from separate pieces.