QUARTZ (AMETHYST) 



483 



Fig. 86. 

 Sceptre-quartz. 



quartz then, there is a long, slender, usually colourless, transparent, or cloudy prism of 

 quartz, attached to the end of which in parallel position is a thicker quartz crystal, which 

 is usually transpai-ent and of a violet colour. 



Crystals of amethyst sometimes attain a considerable size, the largest known being 

 over a foot in length. These very large crystals are rarely quite transparent or uniformly 

 coloured, and are therefore unsuitable for gems ; but there is an abundance 

 of crystals which fulfil these conditions, and which are large enough to 

 supply the market with any amount of rough material. 



With respect to the mode of occurrence of amethyst, we have already 

 seen that crystals of elongated prismatic habit usually occur, like rock- 

 crystal, on the walls of crevices and joints in granite, gneiss, and other 

 rocks. Crystals in which only the hexagonal pyramid or three-faced cube-like 

 termination is developed, have a different mode of occurrence. These 

 usually line amygdaloidal, that is to say, almond-shaped, cavities in a black 

 igneous rock, to which the name melaphyre is applied. The cavities were 

 formed, in their efforts to escape, by the steam and gases imprisoned in the 

 molten igneous rock, and have remained after the solidification of the rock. 

 As long as the rock preserved its fresh, unaltered condition, these cavities 

 remained empty, but with the weathering and alteration of the rock the 

 cavities become wholly or partially filled with secondary minerals. The alteration process 

 is set up by the percolation through the rock of water, which, becoming charged with 

 carbon dioxide and other acids, is enabled to dissolve out many of the constituents of the 

 rock, which are then redeposited in a new form in the cavities. Many and diverse are the 

 secondary minerals formed in this way according to the conditions which prevail in different 

 cases. Amethyst is an important member of this class, and together with it, in the same 

 cavity, agate is frequently found. The secondary minerals thus formed, of course, take the 

 shape of the cavity in which they are deposited, and are therefore referred to as amygdales, 

 the different kinds being distinguished as amethyst amygdales, agate amygdales, &c. The 

 largest may weigh a hundredweight, and the smallest are about the size of a pea. Rocks 

 containing such amygdales are known as amygdaloids. 



When the mother-rock becomes completely decomposed these nests of crystals are to 

 be found among the weathered debris, and are often carried away by running water ; the 

 crystals they contain are finally deposited in the form of rounded grains and pebbles together 

 with other pebbles in the alluvium of streams and rivers. 



At one time the best known amethyst was that which occurs in the cavities of 

 amygdaloidal rocks in the neighbourhood of Oberstein on the Nahe, a tributary of the 

 Rhine. A large amount of it used to be cut in the famous lapidary works of Oberstein, but 

 now the local supplies are for all practical purposes completely exhausted. The cutting of 

 amethyst is still carried on at Oberstein on a large scale, but the rough material has now to 

 be imported. A plentiful supply of fine material at low prices can be easily obtained from 

 other parts of the world, so that the exhaustion of the localities in the Nahe valley has not 

 affected the prosperity of the Oberstein lapidary works. 



Most of the amethyst used for cutting is obtained from Brazil and the neighbouring 

 country of Uruguay. The stones, packed in barrels or in skin sacks, reach Europe, and 

 especially Oberstein, in hundreds of tons, and with them are sent other varieties of quartz, 

 such as citrine, colourless rock-crystal, &c. 



In the State of Rio Grande do Sul, in southern Brazil, and in Uruguay, amethyst occurs, 

 together with citrine and a large amount of agate, in the amygdaloidal cavities of melaphyre. 



