170 On the Porosity of Agates, Calcedonies, Sc. 
and some take on no colour at all. The stones are taken out, 
washed, and placed in an oven. After which they are ground, 
and kept a day in oil, by which some fine cracks are made to 
disappear, and a better polish obtained ; the oil is then rubbed 
off with bran. By this process light grey streaks are 
brought out on some ; and others, according as their porosity 
was greater or less, indicate grey, brown, or black streaks. 
The white impenetrable masses become whiter through the 
loss of their transparency, and many red streaks become 
heightened. 
The so-called Carneole from Brazil, which is wrought in 
Oberstein and Idar in great quantity, costs, on an average, 
about 50 guilders the 100 lb. Those selected with straight 
streaks, as being suitable for cameos, often cost as high 
as 2500 guilders per ewt., receive sometimes the same treat- 
ment as the native stones, and sometimes the process em- 
ployed in colouring carnelian and sardonyx, as I will shortly 
relate. 
They are originally either one-coloured, muddy yellow, 
grey, or contain a variety of shades of such colours, and can 
searcely be called carnelian in their natural state, which 
name is only given to such as are of a red colour. These 
carnelians, when found with streaks, after they have received 
the above-mentioned treatment, form the finest onyx. 
The chemical changes induced by the above-related pro- 
cesses require no detailed explanation. By the placing of the 
stones in hot honey, the latter penetrates into the fine pores of 
the stone ; the vitriolic acid then causes carbonization of the 
animal substance,—and the more the honey in the stone is 
carbonized the darker its colour becomes; and while the 
slightly porous portions become only grey or brown, the more 
porous ones become black. The white and red bands appear 
not to be penetrable to the honey, and it is to the treatment 
alone that we can attribute the increased intensity of their 
colours. Brazilian carnelian contains the oxyhydrate of iron, 
and is generally penetrable in its bands; the red tints are 
destroyed by the carbon, and appear of the colour of a mix- 
ture of grey and black, or most commonly of a dark brown. 
These Brazilian carnelians afford the finest onyxes. 
