GEMS AND PKECIOUS STONES OF GREAT BRITAIN. 
129 
and are found in several parts of Grreat Britain, where they are 
employed in the making of inexpensive ornaments, brooches, 
crosses, seals, &c. The colour of cornelian is also rendered 
more intense by exposure to the suu or the application of heat. 
Other more common varieties, possessing a striped and some- 
times brecciated structure, are known as Agate, and are called 
coralline, moss, ribbon, and fortification agate, according to the 
peculiarities of their internal structures. Such stones are often 
called Scotch pebbles, from being of very frequent occurrence 
in that country. 
It may be here mentioned, that many of the stones called 
Isle of Wight and Brighton pebbles, and assumed to be collected 
on the adjacent beaches, are frequently proved to be agates, 
imported from Grermany, and in some cases have been arti- 
ficially coloured by processes long known to and learnt from 
the lapidaries of Italy.* The colours imparted to them are 
either yellow, blue, dark brown, chocolate, or black, frequently 
giving an onyx or even sardonyx appearance, due to the greater 
or less porosity of the layers of which the agate is composed, 
whereby they are penetrated by the colouring matters in diffe- 
rent degrees. The stones are kept immersed in honey and 
water, or oil, for some time, and afterwards placed in sulphuric 
acid, which carbonizes the organic matter previously absorbed, 
and thus renders them brown or black according to the porosity 
of the stone (see fig. 14). 
Many of the agates are of such great beauty, and so interest- 
ing when considered in relation to their mode of formation, that 
they are fully deserving of being illustrated and considered in a 
separate communication ; a full description of them would be 
quite beyond the limits of the present notice. The stones in fig. 
8 ccc are small examples of agates from Scotland. 
Onyx and sardonyx, which consist of different layers of cal- 
cedony, have both been found in Scotland and Ireland, and have 
occasionally been employed in this country for cameos — a spe- 
cimen of which, from the Griant’s Causeway, so prepared, may be 
also seen in the Jermyn Street Museum. Some of the varieties 
of Jasper, which are agates coloured by much red oxide of iron, 
occasionally form, when polished, pretty ornaments. 
The Bloodstone, or Heliotrope, stated to have been found 
in the Western Islands of Scotland and in Argyleshire, is a 
variety of calcedony which owes its deep green colour to an 
intimate admixture of chlorite (or delessite), whilst the blood- 
red spots whence it derives one of its names are due to the 
presence of the red oxide of iron. Even the common flints are 
frequently used, when cut and polished, as ornamental stones, 
* Die Kimst Onyxe, &c., Noeggerath, Neues Jahrbuch, 1847, p. 473. 
