504 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



C. CHALCEDONY. 



The chalcedony group includes a number of siliceous minerals of compact structure and 

 fine splintery fracture, which are characterised by the possession of a finely fibrous structure. 

 The fibres are always recognisable under the microscope, and sometimes with a simple lens 

 or with the naked eye; they are always very short and their optical characters differ 

 somewhat from those of quartz. Moreover, the specific gravity and the hardness are both 

 less than those of quartz, the former being 2'59-2'60, s.nd the latter H = 6|- ; and 

 in addition the mineral is considerably more soluble in a caustic potash solution than 

 quartz. 



Although chemically chalcedony is, like quartz, pure silica, yet since its physical 

 properties differ it must be regarded as another crystallised modification of this 

 substance. Chalcedony was at one time supposed to consist of a mixture of quartz and 

 opal, which would account for its greater solubility in caustic potash. Microscopic 

 investigation, however, has shown this hypothesis to be untenable. 



Chalcedony being a fibrous aggregate never assumes a regular external form, but it 

 may, and often docs, occur as a pseudomorph after sbme other mineral. That is to say, it 

 assumes the crystalline form characteristic of that mineral by mere replacement of the 

 substance of the crystal, and not in response to an inherent tendency in the molecules of 

 chalcedony to arrange themselves in that particular form. Chalcedony does, however, 

 occur very frequently in rounded, reniform, botryoidal, cylindrical, and stalactitic forms, in 

 which the fibres are arranged perpendicular to the surface. Such masses of chalcedony are 

 built up of very thin concentric layers arranged parallel to the rounded surface. This 

 banding is never wholly absent, but may be indistinct ; it is shown more or less prominently 

 on all surfaces, whether cut or fractured, but is displayed to greatest advantage by a 

 polished surface cut perpendicular to the layers. According as the bands are more or less 

 prominent, the mineral is distinguished as striped or unstriped chalcedony, the former being 

 usually referred to as agate. No very sharp line of demarcation can be drawn, however, 

 between agate and unstriped chalcedony, the one passing insensibly into the other. 



A blow from a hammer will sometimes cause a piece of chalcedony to separate along a 

 surface corresponding to the superimposed layers, the surface of separation being frequently 

 smooth and bright and of course parallel to the original surface. As a rule, however, the 

 layers are so firmly bound together that this separation does not take place, although it is 

 very easy to fracture the mineral by a blow perpendicular to the rounded surface, that is to 

 say, in the direction of the fibres. When crushed, chalcedony always breaks in the direction 

 of the fibres ; this fracture is uneven and finely splintery, and with a feeble lustre inclined 

 to the waxy type. After cutting and polishing the stone acquires a brilliant lustre of the 

 vitreous type. Chalcedony is translucent to semi-transparent, never perfectly transparent. 

 Light passes through it more readily in a direction parallel to the fibres than in one 

 perpendicular to them ; a slab of the mineral cut in a direction perpendicular to the fibres will 

 therefore be more translucent than one of equal thickness cut parallel to the fibres. The 

 different layers of which a specimen of chalcedony is built up vary in translucency, some 

 being almost transparent and others almost opaque. It frequently happens that clear and 

 cloudy layers alternate with each other. 



As a rule, chalcedony is colourless, milk-white, or some faint shade of grey, yellow, or 

 blue, but black and pronounced shades of yellow, brown, red, and green are met with 



