482 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



and patchy specimens are almost worthless as gems, but may be utilised in the manufacture 

 of small semi-ornamental objects. 



The colour of amethyst is very like that of violet corundum, the " oriental amethyst." 

 It compares unfavourably with the latter in one respect, however, since in artificial light it 

 appears of a dingy grey, while under the same conditions the " oriental amethyst " retains 

 its beauty of colour unimpaired. 



Amethyst is dichroic, but this feature is not equally prominent in all crystals, being 

 well marked in some and scarcely appreciable in others. Of the two images seen in the 

 dichroscope one is more reddish and the other more bluish in colour, the difference being 

 more or less well marked according to the character of the crystal under examination. 



The colour of this stone does not resist high temperatures. When subjected to the 

 action of heat it changes first to a more or less pronounced yellow, gradually assumes a 

 gi'eenish shade, and finally disappears. The latter change takes place at 250° C, and the 

 whole process of decolourisation takes place very I'apidly. The change of colour from violet 

 to yellow has a certain practical significance, since thereby the more abundant violet 

 amethyst can be transformed into the rarer yellow citrine. As a matter of fact, many 

 citrines, which are much prized as precious stones, are nothing other than " burnt " 

 amethysts. 



The colour of amethyst has been ascribed to the presence of various substances. The 

 pigment is present in an extremely finely divided state and is mechanically intermixed with 

 the colourless quartz substance, but the individual granules are not recognisable even under 

 the highest powers of the microscope. In patchy specimens the colouring matter is 

 irregularly distributed through the substance of the stone. The pigment has been variously 

 supposed to be potassium ferrocyanide, ferric thiocyanide, or some organic substance, but is 

 most commonly considered to be manganese, which is shown by analysis often to be present, 

 though only in extremely small amount. For example, in a deeply coloured Brazilian 

 amethyst only one-hundredth of a per cent, of manganese was present, while in a stone of a 

 paler colour it was completely absent, so that this evidence cannot be taken as conclusive. 



The form taken by crystals of amethyst agrees in all essentials with that of rock- 

 crystal. Amethyst crystals, too, very often have the prismatic habit of rock-crystal, 

 especially when they occur in the drusy crevices of gneiss and such like rocks. They 

 usually differ, on the other hand, from rock-crystal in the absence of faces other than those 

 of the prism and the terminal faces of the hexagonal pyramid, being in this respect more 

 like common quartz. Crystals of amethyst with the pyramidal termination often occur so 

 closely crowded together on the matrix as to form a columnar aggregate, the individuals 

 of which have irregularly developed faces owing to overcrowding. Of the six terminal 

 pyramid-faces it frequently happens that only three alternate ones are developed, the other 

 three being very small, or even completely absent, so that the crystals sometimes have the 

 appearance of cubes. Such cube-like amethysts are often met with amongst the material 

 sent in large amount from South America for cutting. 



Amethyst crystals are often built of numerous superimposed laminae, of greater or less 

 thickness, in twin position with respect to each other. This peculiar structure is indicated 

 by a fine banding of alternately lighter and darker colouring, and also by the existence of 

 two sets of delicate striations meeting at an angle on the terminal pyramid-faces (Fig. 85d). 

 Delicate striations con-esponding to this lamellar structure are also to be seen on fractured 

 surfaces of amethyst, giving them a kind of rippled (thumb-marked) appearance. In this 

 connection may be mentioned the so-called sceptre-quartz (Fig. 86), a peculiar form 

 sometimes taken by rock-crystal, but much more frequently by amethyst. In sceptre- 



