494 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



bands is due to the bends in the constituent fibres of the stone, and is one of its most 

 characteristic features. The chief beauty of ornaments of tiger-eye, in point of fact, depends 

 upon the aspect of the stone changing with every movement of the wearer. 



In association with tiger-eye there often occurs another stone, which agrees with the 

 former in every respect save that of colour. Thus, it has a finely fibrous structure, a silky 

 lustre, and the same banded appearance of polished surfaces. The physical characters 

 including the hardness are all the same, but the colour instead of being yellow is dark 

 indigo-blue. This blue mineral is also cut as a gem and is known by the name of hawk's- 

 eye. A detailed mineralogical examination demonstrates the fact that it is a colourless 

 transparent quartz, embedded in which are innumerable fine blue fibres, all arranged parallel 

 to each other and perpendicular to the surface of the plate. These fibres consist of the blue 

 asbestos-like mineral crocidolite, a member of the amphibole group, and chemically a silicate 

 of iron and sodium. The colour of sapphire-quartz is due to inclusions of the same 

 mineral, but in this case the fibres are not parallel but arranged quite iiTegularly. 



In their structure and mode of origin tiger-eye and hawk's-eye are very closely 

 connected as will presently be seen. An examination of a piece of hawk's-eye often results 

 in the observation that the stone is not of one uniform blue colour, but that portions of it 

 are yellow. These blue and yellow portions may be present in equal proportions, or the 

 blue may be present in larger proportion than the yellow, or the opposite conditions may 

 prevail. Now these facts clearly indicate that the yellow substance, which in every respect 

 is identical with tiger-eye, has been formed by the alteration of the blue hawk's-eye. 

 Moreover, it appears that it is the crocidolite which has undergone this alteration, its 

 constituents, with the exception of silica and iron, having been dissolved and carried away. 

 The silica which remains behind retains the fibrous form of the original mineral, but assumes 

 a yellow colour owing to the deposition between the fibres of a small amount of hydrated 

 oxide of iron, this also being an alteration product of crocidolite. When in any specimen 

 of hawk's-eye the alteration process has just begun, there will be little yellow patches 

 scattered here and there, which will increase in extent as the alteration proceeds, until 

 finally the bhie hawk's-eye becomes wholly converted into yellow tiger-eye, the fibrous 

 structure of the original stone remaining, however, unchanged. 



Beside stones which show every stage of transition between hawk's-eye and tiger-eye, 

 there are others which are intermediate in character between hawk's-eye and crocidolite, and 

 tiger-eye and crocidolite. Like hawk's-eye and tiger-eye, crocidolite occurs in parallel-sided 

 plates or slabs, with its abestos-like fibres perpendicular to the surfaces of the plate. In the 

 fresh condition this asbestiform mineral is of a blue colour, but when altered it is yellow, 

 owing to the oxidation and hydration of the ferrous iron it contains to hydrated ferric 

 oxide. This yellow alteration product of crocidolite, consisting of a mechanical mixture of 

 silica and hydrated iron oxide, has been given the name of " griqualandite." By the 

 infiltration of silica between the fibres of crocidolite tiger-eye and hawk's-eye are formed, so 

 that it is possible to regard these minerals as silicified crocidolite or as pseudomorphs of 

 quartz after crocidolite. The quartz assumes the structure of the original mineral, the 

 fibres of which remain embedded in it. If these remaining fibres are fresh and unaltered, 

 the resulting mineral is blue hawk's-eye ; if, on the other hand, they consist of the yellow 

 alteration product, it is yellow tiger-eye. It is also possible, however, that the alteration of 

 the crocidolite, which is accompanied by the change in colour from blue to yellow, takes 

 place in part at least after, instead of before, the silicification of the asbestiform mineral. 

 Because of the relation which exists between crocidolite and tiger-eye the latter is frequently 

 referred to in the trade as crocidolite. 



