36 
POPULAR SCIENCE REVIEW. 
cameo-worker, who skilfully engraves a subject in the white 
layer, which then stands out upon a dark-coloured ground* 
If the lower stratum, instead of being black, be brown or red- 
dish, the stone is known as a sardonyx : large numbers of such 
stones are cut for setting in rings. The reddish tint of the 
sardonyx and of the camelian may he readily developed arti- 
ficially, and the process of “ burning ” by which this is effected 
was indeed known in Germany long before the methods of 
colouring onyxes were patent. It had often been observed that 
greyish-coloured agates, after long exposure to sunshine, be- 
came reddened, and the effect of artificial heat in developing 
the colour had likewise been accidentally observed. Experi- 
ments were tried in 1813, and since then the stones have been 
systematically burnt whenever carnelians are required, as has 
indeed been practised for ages in the East. The German work- 
men expose the stones for several weeks to the heat of an oven, 
the temperature being at first very gentle, and then gradually 
raised. When all moisture has been thus expelled, the stones 
are moistened with sulphuric acid, and again exposed to heat, 
the temperature being this time slowly raised to redness. The 
reddened stone must of course be allowed to cool very gradually. 
In 1845 an Idar manufacturer introduced a method of 
colouring stones bright blue ; but this process, unlike those pre- 
viously described, produces an effect quite unknown among 
natural stones. Commonly, the agate is steeped first in solution 
of a ferric salt — a per -salt of iron — and then in ferrocyanide of 
potassium, or yellow prussiate of potash, whereby a precipitate 
of Prussian blue is thrown down in the pores of the stone. 
Other methods are employed, but these will suggest themselves' 
to any chemist ; in fact, almost any process yielding a blue pre- 
cipitate may be applied. 
About the year 1855 a green colour was introduced, and 
chalcedony was thus tinted to resemble the natural chrysophrase.. 
This colour is produced by the use either of chromic acid or of a 
salt of nickel. Yellow is also a favourite tint among the 
Oberstein workers, and is commonly obtained by steeping the 
stones in hydrochloric acid. Of late years various fancy colours 
have likewise been employed, and even the aniline dyes have 
been pressed into the lapidary’s service. Such tints are, how- 
ever, fugitive, and are certainly to be eschewed as utterly un- 
natural, and therefore to most mineralogists little short of 
repulsive. 
It is unnecessary to follow any of the minor branches of the 
agate industry, but in dismissing the subject let it not be for- 
gotten that it is an industry which, in the neighbourhood of 
Oberstein and Idar, gives employment to some three thousand 
hard-working and contented people. 
