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This is a mineral of various colors, but generally of a faint pink, green, or violet tinge, which is sometimes seen in jewelry but more often in decorative works. In jewelry, the violet blue variety, known as Blue John, is often used, particularly in certain districts where the stone is mined. But greens and pinks, fash­ioned into beads or carved into ornamental figures (generally by the Chinese) are also on the market. Although really too soft to

be used as a jewel stone, the hardness being only 4, fluorspar pos­sesses certain outstanding qualities which render it deserving of notice.

Chemically, this mineral is calcium fluoride, the various colors perhaps being due to colloidal matter since they are very quickly altered by heating. At a temperature below red heat, a soft green­ish light is emitted, but before the blowpipe decrepitation takes place and the substance fuses into an enamel. This property is of consideration in the use of fluorspar as a flux, in the capacity of which it has been used over a long period in various metallurgical processes, such as in the smelting of lead and copper. Its name is even derived from the Latin fluo, meaning "to flow."

Natural crystals are most common, these being perfect cubes and often twinned, but not wholly transparent. Many are large and complete in shape, sometimes eighteen inches in length, and they possess perfect octahedral cleavage. Since the crystals will fall into the cubic system, refraction will be single, the general read­ing being 1.433. Specific gravity is 3.18.

It must be noted, however, that two shades of color in the one specimen may be seen sometimes. For instance, blue and green may be discerned with the naked eye; this is thought to be due to the action of the ultraviolet rays of light, and the outstanding property possessed by fluorspar of appearing differently colored by reflected and transmitted light is known as fluorescence. A vast range of colors have been found, but shades are generally not strongly pronounced.

Single refraction and softness will alone distinguish it from most similarly colored minerals. For instance, the pink looks something like rose quartz, the mauve like amethyst quartz, and the greens like poor quality emerald. But fluor does not take a high polish, and it gives the impression of being a soft stone. Per­fect cleavage makes it easy to work, although at the same time the material is easily chipped and broken. Beads of fluorspar, no doubt of a great age, have been discovered in various parts of the world.

Blue John, the mauve lined variety said to be exclusively found in the Peak district of Derbyshire, is one of the few stones found in England which is sufficiently attractive to be used in jewelry. It was mined in Derbyshire during the Roman occupa­tion of this island, but good material has now become very scarce, and the only mine from which stones are still cut is situated about eight miles from Buxton and about a mile from Castleton. The latter village is the center for working the stone into cabochons and larger ornamental pieces. The whole district contains inter­esting caverns and caves; some are rich in fine specimens of nat­ural stalactites and stalagmites, and in one cave an underground river of over half a mile in length is to be found.

Although pale and almost transparent green, blue, pink, gray, yellow, and even colorless specimens are found in many lands, the rich mauve of the Derbyshire Blue John is outstanding. The material is often veined and lined, and it occurs here in rocks which are rich in lead ores. In fact, the early mining for lead must have been the reason for its discovery, for this district is full of old lead mines, now almost all disused. The name Blue John is a miner's term for the mineral, a pocket of zinc blende, which is occasionally found with the stone, having the name of Black Jack.

Blue John is sometimes found in a color which is too dark for ornamental use. The most prized shade is that known as Bull Blood. The alternate layers of deep purple and blue are known as Bull Beef, and so scarce are good specimens that large vases and other objects are now being made by cementing together various matched sections of the stone, in contrast to the older works, which were cut from one piece of stone.

The cutting and polishing is best carried out by hand, and this needs long experience to be successful, despite the comparative softness and easy cleavage of the material. Cracks easily develop, and the secret of polishing is retained by the local workers. But it seems that the rough stone is first gently heated and then steeped in resin before being worked; this avoids chipping and also accentuates the purple color. The addition of sulphuric acid decomposes fluorspar, hydrochloric acid being formed.

As large pieces of suitable material are now very scarce, the smaller pieces once discarded are now being re-examined and cut into ring and brooch cabochons. Cups, vases, and larger ornaments are now seldom fashioned, and most of these large specimens which one sees are the products of former work.

Worked examples in this stone have been found in the ruins of Pompeii, and their quality suggests that they must have originated from the Castleton mines in Britain. The vas murrhinae, described by Pliny, are thought by many to have been made from the same material. If the vas murrhinae were of fluorspar, then the mineral was highly valued by the Romans. Six were taken from the treasures of Mithridates and placed by Pompey in the temple of Jupiter in the Capitol. After the defeat of Antony, Au­gustus carried off from Alexandria a murrhine vase which he con­sidered the most precious of his trophies. Nero, it is said, gave 300 talents (more than $112,000 or £40,000) for a single cup in this material. Another finely carved piece may be seen in Naples, while the largest vase of Blue John known is in the possession of the Duke of Devonshire. This latter could be viewed until recently in the sculpture gallery at Chatsworth House, not far from Castle-ton.

The ordinary variety occurs in veins, gneiss, limestone, or sandstone. It is found associated with tin ore in Saxony and in Cornwall, England, with lead ores in Derbyshire, Cumberland, Northumberland and Durham, and with silver ores in certain parts of Norway and Germany. It is also found in the Transvaal and in Manchuria. Pale colored beads have been unearthed in some ancient ruins near Lake Titicaca in Bolivia, so the occur­rence and use of fluor is fairly widespread.

Until the end of the last century, common fluorspar was largely used for making hydrofluoric acid and in the manufacture of colored glass. In recent years, much is added as a flux to furnace charges in order to give fluidity to the slag. It also assists in re­covering such impurities as sulphur and phosphorus. The iron industry also uses this material as a flux, and in the ceramic in­dustry it is used to produce enamel coatings for household wares. Commercially, the Derbyshire deposits are now not much used since the principal importer, the U.S.A., has developed its own deposits in Illinois and Kentucky, but these are also now being replaced by imports from Mexico.