530 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



The colour of green fluor-spar or "false emerald" is sometimes very fine and 

 approaches to that of the true emerald. Crystals from certain English localities, from the 

 porphyry of Petersburg near Halle, and from the metalliferous veins of Badenweiler, are 

 remarkable in this respect. A recently discovered occurrence is that at Macomb, in 

 St. Lawrence County, New York, where thousands of beautiful green crystals, with a total 

 weight of fifteen tons, were found in a single large cavity. 



Blue fluor-spar, or " false sapphire," of a very dark colour, is found in the tin mines 

 of the Erzgebirge, and also in salt mines at Hall in the Tyrol. The cubes of fluor-spar 

 from Alston Moor in Cumberland have the peculiar property of appearing by transmitted 

 light of a fine green, and by reflected light of a dark blue colour. The phenomenon from 

 its occurrence in fluor-spar is known as fluorescence. Stones of this description are 

 sometimes mounted t, jour in pins, rings, &c., in order to display their fluorescence. Other 

 blue crystals, especially when of a dark shade of colour, have a perceptible violet tinge, 

 while others are lighter in shade and of as pronounced a violet colour as amethyst itself. 

 Magnificent crystals of violet fluor-spar, or " false amethyst," are found in the lead mines 

 of Weardale, in Durham. 



These various colour-varieties of fluor-spar are cut in the forms adopted for the 

 precious stones they respectively resemble. They acquire a good polish, but require great 

 care both in cutting and in wear, since the lack of hardness and the perfect cleavage 

 renders them liable to be scratched or cracked and splintered. They are never worth 

 very much, and can always be distinguished from the valuable precious stones they 

 may resemble in colour by their lack of hardness, their specific gravity, single refraction, 

 and by the fact that, in correspondence with their crystalline form, they are not 

 dichroic. 



Though little worn as a gem, fluor-spar is somewhat extensively used as a material for 

 ornamental objects such as bowls, vases, candlesticks, letter-weights, or even for columns, 

 mantel-pieces, &c. The so-called spar-ornaments of this description are manufactured 

 chiefly in England of the variety of fluor-spar known as " blue John," which is found in 

 large amount and of fine quality at Tray Cliff\, near Castleton, in Derbyshire, This is 

 massive, coarsely-grained material of a dark blue colour tinged with violet, frequently 

 intersected with white and yellow bands. The stones are ground into the desired form 

 and can be worked on the lathe ; the process, however, needs great care, for, owing to the 

 brittleness and easy cleavage of the mineral, it is very liable to splinter. Since the 

 commencement of the industry in 1765 many matters of technique have been learnt 

 by experience. The fluor-spar is now, for example, impregnated with resin in order to 

 make it tougher and less Table to splinter. Material so treated can be fashioned into 

 vessels the walls of which are not more than 1 or \\ lines in thickness, this excessive 

 tenuity being necessary because of the depth of colour of the mineral. When " blue 

 John" is exposed to a temperature just below a red-heat the dark violet-blue colour 

 changes to a beautiful amethystine violet, which is not shown by stones in their natural 

 condition. The operation needs to be performed with great care, for not only is the stone 

 liable to crack, but if the temperature is too high it will be completely decolourised. 

 Owing to the comparative abundance of the mineral and the ease with which rough 

 material is obtained, articles fashioned of fluor-spar do not cost much more than the value 

 of the labour expended upon their production ; this is not inconsiderable, owing to the 

 difficulty of the work. 



It has been suggested that the murrhine vases of the ancient Romans were made of 

 fluor-spar, but there is no conclusive evidence to support this, and it is more probable, as. 



