522 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



as the style. The actual work of agate-engraving is performed principally in Paris and 

 Italy, but the plates of onyx used by the engravers are prepared at Oberstein. In Italy 

 onyx is largely replaced by plates cut from the thick shells of certain marine molluscs, these 

 shells, like carnelian-onyx, being built up of red and white layers. They are very much 

 softer than agate, and can be cut with much less labour. 



It is not only stones with plane surfaces, however, which are cut as cameos, for there 

 are a few onyx vessels in existence decorated in this way, the body of the vessel being 

 formed by a layer of one colour and the raised figures cut in a layer of another colour. 

 The famous onyx vase preserved in the collection at Brunswick is a monument to the skill 

 of ancient artists in this direction. 



The artificial colouring of stones is an important branch of the agate industry at 

 Oberstein, especially since the supplies of rough material have been obtained principally 

 from Brazil. Most of the agate found in nature is of a dingy grey colour, and, until the 

 discovery of the methods by which it may be coloured, was totally unfit for cutting. The 

 first step was to colour the stones black, this art being stated to have been imparted to an 

 agate merchant at Idar by a customer at Rome, who paid an annual visit to Idar for the 

 purpose of buying onyx. The art, which had been long known in Rome, was afterwards 

 extensively practised at Oberstein, and in course of time was developed and improved. 

 Not only was a black colour imparted to stones but also brown, yellow, blue, green, and 

 red. 



The artificial colouring of agate is possible because of the porous nature, not only of 

 this stone but also of all other varieties of chalcedony, by virtue of which they absorb any 

 coloured liquid in which they are immersed. All specimens of agate are not equally 

 porous, neither are all layers of the same piece ; some are very porous, readily absorbing a 

 large quantity of coloured liquid and thus acquiring an intense colour, while others are 

 quite or almost non-porous, absorbing colouring matter very slowly and with great difficulty, 

 so that they are only faintly coloured. Specimens answering the former description are 

 described by lapidaries as " soft " and the latter as " hard." In this sense the agate found 

 at Oberstein is " harder " than that occurring in Brazil, which is specially suitable for 

 artificial colouring. In Brazilian agates, however, the outer portion of the amygdale, the 

 so-called skin, is difficult to colour, and the milk-white layers are practically non-porous 

 and cannot be coloured at all. If the stone is allowed to remain long enough in the 

 colouring liquid, this will penetrate to the innermost parts of the stone, so that the whole 

 mass, and not merely a superficial layer, is coloured. It has been observed that the 

 absorption of liquid takes place much more rapidly radially, that is to say, perpendicular to 

 the layers, than along the layers, this being due to the radial extension and arrangement of 

 the pores already described. 



The principle of the colouring process is thus quite simple, although in practice there 

 are several points which require attention if a good result is to be obtained, and the work 

 is entrusted only to experienced workmen. The most essential qualification for the work is 

 a familiarity with the appearance and character of different types of stones, but even with 

 this qualification it is not always possible to foretell the result of immersing a given stone 

 in a given liquid. It may happen, for example, that after the immersion in the colouring 

 liquid of a number of stones apparently equally suitable for the purpose, some will be 

 green and others blue. The complete process is still in many cases jealously preserved as a 

 trade secret, as was the case at first with the process of staining the stones black. 



The latter process has been known in Oberstein since 1819. The stone to be coloured 

 black is first washed clean and dried without the application of heat, and then immersed 



