QUARTZ. 



ffrvtd on the roofs and fides of artificial excavalions, and 

 pad'ages which have been undiRurbcd for feme years. Ac- 

 cordino-toGhaptal, aquartzofeorafihceouspaiteisformedby 

 tranfudation on ferruginous rocks at Chamillat, near Planches 

 les Mines, in tranche Compte, and where this is wafhed and 

 depofited by water, rock cryftals are formed. Sihceous 

 ftalaftites have not unfrequently been feen coating the wood 

 which has been left in mines. 



Pfcudomorphous cryltals of quartz, or falfe cryilals of 

 this mineral, are frequently found in mines ; they are tar- 

 nifiied and opaque, and their edges are blunted. They 

 alTume the forms of cubic cryftals of fluor, and the pyra- 

 midal and other forms of calcareous and other cryftals. 

 Thefe falfe cryltals are evidently moulded in the cavities 

 which the former have once occupied. Quartz alio occurs 

 fiUing up the cavities of (hells in a fimilar manner, and the 

 pores of wood and other organic fubftances. 



Celluhr quartz with polyhedral cavities, appears to have 

 been formed round cryftals of other minerals which are fub- 

 fequently decompofcd. The internal cryftals are fre- 

 quently thofe of the metallic fulphurets. Brongniart fays 

 this conclufion is founded on direft obfervation, as the re- 

 mains found in thefe cavities are fulphur, native gold, and 

 oxyd of iron, all fubftances which originally formed part of 

 the decompofed fulphurets. Such is the cellular quartz at 

 Schemnitz in Hungary, and Joachimfthal in Bohemia. The 

 cellular quartz at Berezof, in Siberia, is fo porous that it is 

 lighter than pumice. 



Roch cryfial, or Mountain cryjlal, is the pureft variety of 

 quartz, differing from common quartz by its tranfparency, 

 and the more reguUar form of its cryftals ; the frafture is 

 alfo more perfectly conchoidal. According to fome analyfes, 

 rock cryllal contains 98 parts of filex, with 2 parts of 

 water. Bergman found in one fpecimcn only 



Silex - 93 

 Alumina 6 

 Lime - I 



Amethyjl is a purple variety of rock cryftal. See Ame.. 



THYST. 



Catrngorumjlonis are rock cryftals, from the mountains 

 of Cairngorum, in Scotland. They are valued by jewellers 

 on account of their purity and colour. Clove-brown quartz 

 cryftals are known by the name of the fmoke topaz. Ac- 

 cording to Karften, the fpecific gravity of this variety is 

 2.88. Coloured rock cryftals lofe their colour, when care- 

 fully cxpofed to a gentle heat, but retain their tranfparency. 

 The yellow and orange-yellow varieties are the moll 

 efteemed. 



Milk quartz, called by fome mineralogifts rofe quartz, 

 and by others the Bohemian ruby, as it is frequently of a 

 beautiful rofe-red colour, which it derives from manganefe. 

 By expofure to the light the intenfity of its colour is dimi- 

 niftied. It is ufed in jewellery, and takes a good poHfli ; it 

 is fometimes fold for the ruby, but is lefs hard, nor has it 

 the tranfparency and brilliancy of that gem. Rofe quartz 

 is found at Rabenftein, in Bavaria, in confiderable maftes, 

 and in a vein of manganefe which traverfes a coarfe-grained 

 granite. It occurs alfo in Sweden, Greenland, Saxony, in 

 the ifland of Coll, one of the Hebrides, and in Ireland. 

 Some varieties are of a milk-white colour, others pearl-grey. 



Iridefcent quartz, Quar% hyalin irife, prefents on its fur- 

 face, or in its interior, the various colours of the rainbow : 

 it fometimes derives this property from a thin pellicle of me- 

 tallic oxyd which covers the furface, and fometimes from 

 minute filTures in the fubftance of the cryftal. Thefe colours 

 may be given to quartz by heating it and expofing it fud- 

 denly to a cold temperature. 



ytvanturini quartz (fee AvAKTuniNE) owes its brilliancy 

 to miimte particles of mica difl'cminalcd through it, and 

 fometimes to minute fraftures. Avanlurines may be formed 

 ai tificially, in fome varieties of quartz, by heat. 



Blue quartz, Qtiarz hyalin faphaln, has been found in 

 Spain, Bohemia, and Bavaria. 



Black quartz is found, mixed with carbonate of lime, iit 

 the department of Here in France, in Bohemia, and at 

 Cappe Nuova, on the road to Sienna. Brongiriart Mine- 

 ralogie. 



Cat's-eye, Quarz hyalin chatoyant, fo called becaufe it re- 

 fle£ls a pearly variety of colours, according as the light falls 

 upon it in difterent direftions : this property is owing to the 

 fibrous texture of this mineral, which may be obferved in 

 the bell charatlcrized fpecimens. It contains 95 parts of 

 filex, and according to Brongniart is a variety of quartz, and 

 ought not to be clafled with felfpar. The geological fitu- 

 ation of this Itone is not known. The cut and polifhed 

 fpecimens feen in cabinets come from Malabar and Ceylon. 

 It is faid, alfo, to occur in Egypt and Arabia. 



Green quartz, Prafe, Quarz hyalin ttcrd obfcur. The colour 

 is a leek green ; it is feldom cryftallized ; the form of the 

 cryftals are thofe of common quartz. Prafe has been faid 

 to owe its colour to an intimate intermixture with aClyno- 

 lite, but Klaproth is of opinion that this is not well afcer- 

 tained. 



Sinople is clatFed, by fome mineralogifts, with quartz. It 

 is of a deep blood-red colour, and perfectly opaque, or 

 barely tranfmits light on the edges. It refembles red jafper, 

 but has a vitreous luftre, and conchoidal frafture, and is 

 fometimes cryftallized. 



Ferruginous quartz confifts of fmall cryftals, or of granu- 

 lar quartz, intimately mixed with oxyd of iron, and has a 

 brown ochre-yellow, or red colour. From the iron which 

 it contains it becomes magnetic when heated. It is harder 

 than pure quartz. 



Certain properties, befide colour, have given names to 

 fome varieties of quartz. 



Fat quartz is fo called becaufe it has a greafy appearance, 

 as if the furface had been rubbed with oil. 



Fetid quartz differs from common quartz, by emitting an 

 odour, when rubbed, hke that of fulphuretted Hydrogen 

 gas. When this foflil is heated below a red heat, it lofes 

 its fetid odour on cooling, and when plunged into water be- 

 comes tranfparent. When fetid it is nearly opaque. It is 

 found in the vicinity of Nantes and Chantiloub, in the de- 

 partment of Haute Vienne, forming a conftituent part of 

 the granitic mountains of that diftrift. It occurs alfo in the 

 ifland of Elba. 



Elajlic quartz is a filiceous fand-ftone, found in the Bra- 

 zils, compofed of oblong lamina of quartz, arranged in 

 one direftion, and fo interlocked together as to form a kind 

 of hinge with each other, from whence it poileffes a certain 

 degree of pliability, hke that of a ftiff piece of leather. In 

 appearance, elaftic quartz refembles fome of the flaty fand- 

 ftones in the northern counties of England. According to 

 Klaproth its conftituent parts are 



Silex - • 96.50 



Alumine - - 2.50 



Oxyd of iron and lofs i 



The ufe of quartz in the arts is principally confined to 

 the manufaftures of glafs, enamels, porcelain, and earthen- 

 ware. The finer cryftaUizations are employed by the jewel, 

 lers, and before the difcovery of glafs, ornaments and veflels 

 of great value were made by the ancients from rock cryftal. 

 Stones in which quartz forms the principal ingredient, are 

 beft fitted for purpofes of durable architefture, and in the 



economy 



