34 
POPULAR SCIENCE REVIEW. 
and the engraving of cameos, on onyx, are branches of agate- 
working which are largely practised at Oberstein, but which we 
have no space here to describe. It remains, however, to notice 
one of the most interesting departments of the industry. 
Beautiful as agates unquestionably are in their natural state, 
their beauty is, in the,., judgment of most people, greatly en- 
hanced by the artificial processes of colouring to which the stones 
are now almost universally subjected by the Oberstein workers. 
Hot that the art of staining is by any means a modern discovery. 
It was, in fact, known to the ancients, and the matter did not 
escape the notice of the omnivorous Pliny, though his description 
is obviously imperfect.* He tells us that the Arabian stones are 
purified by leaving them for seven days and seven nights in honey. 
How the stones might be left in honey till Doomsday without their 
tint being in any wise improved, and yet not a word is said with 
respect to any further treatment. If a stone, which has been 
steeped in honey, be placed in sulphuric acid, the acid entering 
the pores of the stone decomposes the saccharine matter which 
has been absorbed, and a deposit of carbon is thus thrown down 
in a finely divided form in the interstices of the stone, producing 
a deep black colour. To believe that the ancients stained their 
agates in this way is to assume that they were acquainted with 
oil of vitriol ; but as it is generally believed that this acid was 
first obtained by Basil Valentine in the fifteenth century, some 
writers have suggested that the Boman stone-workers availed 
themselves of the sulphuric acid naturally exhaled in certain 
volcanic districts, whilst others again have maintained that the 
sugar was charred by simple exposure to heat. In whatever 
manner the ancients effected the colouring, it is certain that 
the Italian cameo-workers have always been familiar with a 
process of staining, and these workers were in the habit of 
visiting Oberstein, from time to time, for the purpose of pur- 
chasing the finest onyxes, which they took back to Borne, there 
to be stained and engraved. The German workers, who sold the 
uncoloured stones, remained, however, entirely ignorant of the 
process until the year 1819. It then happened that a native of 
Idar and one of the Boman stone-engravers got into difficulties 
in Paris, and were imprisoned together ; during their confine- 
ment they became communicative, the conversation frequently 
turned upon agates, in which they had a common interest, and 
the secret escaped from the loquacious Italian. Shortly after- 
wards it was conveyed to Oberstein, and, once out, soon became 
* See Noggerath’s paper, “Die Kunst Onyxe, Carneole, C talced one, und 
andere verwandte Steine zu farben, zu Erlauterung einer Stelle des Plinius 
Secundus.” (“ Nenes Jahrbuch,” 1847, p. 473.) The passage referred to 
is in Pliny’s “ Nat. Hist.,” bk. xxxvii. cap. 75. 
