On the Porosity of Agates, Calcedonies, Sc. 167 
applied to the surface, and sometimes made to penetrate 
slightly into them. The processes have been frequently de- 
scribed, but it remained unknown how to render the various 
quartzes included among the gems of the ancients penetrable 
to colouring fluids. 
Within the last twenty or twenty-five years the processes 
of the agate-cutters of Oberstein and Idar have reached such 
perfection that they are able not only to bring out and 
heighten the natural colours of caleedony, onyx, carnelian, &e., 
which are sometimes faint, but also to render them entirely 
penetrable to colouring fluids by which the beauty and variety 
of the stones is much increased. 
This colouring process was at first a secret, known only to 
a few agate-dealers in Idar. It was eagerly sought after by 
Roman stone-cutters (as the lapidaries of Oberstein Say), 
‘who bought up all the onyxes. The secret seems at length 
to have been discovered by some of the foreigners, or been 
bought up. 
This art arises out of the property which the fine layers ~ 
of calcedony, although exhibiting only faint differences of 
colour, possess of becoming variously coloured by the appli- 
eation of colouring fluids. By this process very mean-looking 
slightly-coloured stones can be turned into very fine onyxes, 
&e., which by their various bands of colour afford materials 
for cameos ; and at least the beauty and designs of the agates 
intended for other purposes are much increased. 
There is a method by which the agate-dealers of Ober- 
stein and Idar determine the fitness of the crude minerals 
for the colouring process; at least to value them before pur- 
chasing them from the diggers. They break off a thin por- 
tion of a seemingly useful mass, and after moistening it with 
the tongue, observe whether the absorption of the moisture 
by the alternate bands takes place at regular intervals ; if so, 
it is then deemed fit to be coloured as an onyx. This proof 
is not always decisive of the value of the minerals, yet it af- 
fords a fair criterion to the dealers to go by before they buy 
valuable pieces from the diggers. 
Large balls of calcedony, in which many fine bands oceur, 
especially if the rest be of a red colour, are much prized. 
