GEMS AND PRECIOUS STONES OF GREAT BRITAIN. 
127 
quartz does uot possess that brilliant lustre whicb is due to the 
highly refractive powers of the diamond, still to the ordinary 
observer it presents an appearance so similar to it, that it is 
frequently mistaken for that gem. 
In consequence of this, the purer quartz crystals have been 
extensively employed in jewellery, and thus have given rise to the 
names of Bristol, Cornish, Welsh, Irish and Bohemian diamonds, 
from the localities where the mineral occurs. 
When the otherwise colourless crystals of quartz have a more 
or less pronounced hue imparted to them by other substances, 
we have a great variety of often very beautiful stones, largely 
used in personal decoration, such as amethyst, blue, rose, and 
aventurine quartz, cairngorm, &c. 
The very beautiful and well-known stone amethyst is simply 
quartz coloured by a very minute quantity of oxide of manganese. 
This variety of quartz is invariably found crystallised in mineral 
veins or cavities of rocks, and frequently in geodes. In Eng- 
land it has been found of a fine deep colour at the United 
Mines and Wheal Unity in Cornwall, and in geodes or potato- 
stones in Somersetshire. In Ireland, at Achil Island, Mayo, 
in fine translucent crystals, occasionally attaining eight or ten 
inches in length, and several other localities. In Scotland 
it occurs in the Hebrides, in Lewis and North Uist, and in cavities 
of agate in Fife and Perthshire. The figs. Saa and fig. 86 are 
specimens of deep and pale-coloured amethysts. 
The name amethyst {afji,WuGTo$) was applied to this stone in 
consequence of the property ascribed to it by the ancients, and 
recorded by Aristotle, as a preservative against intoxication. 
Fig. 2 shows a small group of crystals in part of an agate geode. 
The amethyst has always been a favourite stone independently 
of the many virtues attributed to it, besides that of preventing 
intoxication. It is known as the Bishop’s Stone from its being 
worn as a ring by the Koman Catholic bishops, just as the green 
variety of tourmaline is said to be used as a ring-stone by the 
clergy of Brazil, from which country a large part of the 
amethysts used in jewellery are obtained. When of a fine and 
uniform colour, the amethyst is still perhaps as much esteemed 
at the present time as it was by the ladies of ancient Eome. 
The yellow and brown varieties of quartz are in Scotland 
termed Cairngorm, from the celebrated mountain locality of that 
name, and which, from the earliest periods of Scottish history 
to the present time, have been regarded as ornamental stones of 
almost national character. 
In the box (fig. 8), the central stone is a fine yellow cairn- 
gorm, whilst the entire bottom is formed of a plate of similar 
stone but of more pronounced colour. Fig. 10 is a representation 
of a beautiful specimen of a rich yellow-brown tint. Fig. 11 
