QUARTZ (CHALCEDONY) 505 



octasionally, and blue as a rarity. Some specimens are of one uniform colour throughout, 

 but more frequently the different layers of the stone are differently coloured, the colour, 

 however, being uniform throughout the same layer. This contrast in the colouring of the 

 layers has tlie effect of bringing much into prominence the banded structure of 

 the stone. 



Chalcedony being somewhat porous it can be artificially coloured with ease simply by 

 immersion in a fluid containing colouring matter in solution. The liquid is sucked up into 

 the pores of the stone, and as the stone dries and the liquid evaporates the colouring matter 

 is deposited in these pores and imparts its colour to the whole mass. So porous are some 

 specimens of chalcedony that they will adhere to the tongue owing to the rapidity ^^■ith 

 which the moisture of that organ is absorbed, and when placed in water the expulsion of the 

 air-bubbles from the pores is so rapid as to cause a hissing noise. Not infrequently the 

 pores, in the form of round cavities or elongated canals, can be recognised under the 

 microscope, or even with a simple lens. J3iff'erent layers of the same specimen are not 

 equally porous, some will absorb the colouring matter with great rapidity, while others will 

 be scarcely coloured at all. 



The intense colours shown by certain specimens of natural chalcedony have in many- 

 cases been assumed after the formation of the mineral, by a natural process analagous to 

 the artificial method. 



The artificial colouring of chalcedony has become quite a prominent feature of the 

 agate-cutting industry. It is carried on to such an extent that there is actually more of the 

 artificially coloured material sold than of the natural stone. The subject will be treated in 

 more detail, however, when the working of the stone is described. 



Chalcedony occurs in layers with a mammillated surface coating the surfaces of rocks in 

 cracks and crevices, and in favourable positions in the form of stalactites. It is found usually 

 in volcanic rocks, which, when chalcedony is present in any amount, are always in an 

 advanced stage of decomposition. Like opal and amethyst, chalcedony is thus an alteration 

 product of the rock in which it occurs, and when the last stages of decomposition have been 

 reached the chalcedony is set free and lies loose in the soil in isolated fragments, which are 

 often carried aMay by running water and finally deposited as pebbles in the beds of rivers 

 and in other alluvial deposits. 



Several varieties of chalcedony are recognised by lapidaries, the distinctions between 

 them being based on differences in colour and the mode of its distribution, diflferences which 

 are not, however, very sharply defined. It is, moreover, sometimes difficult to decide whether 

 a given specimen should be referred to chalcedony, jasper, or hornstone. Mineralogically 

 chalcedony is determined mainly by the finely fibrous structure, but lapidaries classify the 

 compact quartz minerals according to translucency alone, all the perfectly opaque varieties 

 being referred to as jasper, and all those which are more or less translucent as chalcedony. 



The vai'ieties of chalcedony here distinguished are as follows : 



Common chalcedony of a faint uniform colour. 



Carnelian of a uniform red colour, with which is included brown Sard. 



Plasma of a uniform dark green colour. This, when spotted with red, is known as 



heliotrope, or blood-stone. 

 Agate with a prominent banded structure. The well-known omjx is included under this 



" striped chalcedony.'' 



The sub-classification of these varieties will be given below. 



