508 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



through the thin wall of translucent chalcedony, and can be heard when the stone is shaken. 

 These nodules are now found in masses of clay, but they were originally formed in solid 

 rocks of volcanic origin, which in course of time have been decomposed and weathered to 

 clay. The mode of occurrence of these enhydros is the same as that of ordinary 

 chalcedony, and they must be regarded as chalcedony amygdales which have been only 

 partially filled with mineral matter. Throughout the weathering of the rock they 

 retain the form of the original rock-cavity, and when the process is complete can be 

 extracted from the soft clay without injury, which would be impossible while they are 

 embedded in solid rock. Such nodules of chalcedony containing water, and no bigger than 

 a nut, were first obtained from Monte Tondo in the Colli Berici, near Vicenza, in northern 

 Italy, where they occurred at rare intervals in weathered basalt. They were known to the 

 ancients, and are mentioned by Pliny. At the present time they are obtained in considerable 

 numbers from Uruguay, where they occur with agate in weathered amygdaloid, the largest 

 being half the size of a man's fist. 



When such enhydros are exposed to dry air, it sometimes happens that the water 

 contained in them slowly evaporates, but is re-absorbed when the stones are immersed in 

 water. This furnishes still another instance of the porous nature of chalcedony, and also 

 throws much light on the origin of agate amygdales to be discussed below. 



Enhydros are sometimes polished, although they are used so little as gem-stones. 

 Stones of sizes less than a nut are chosen for this purpose ; their surfaces are smoothed and 

 polished very carefully in order to avoid breaking the nodule, and the movement of the 

 enclosed liquid can then be clearly seen. The stones thus prepared are mounted in rings, 

 pins, &c., and are worn more as curiosities than for the sake of any intrinsic beauty they 

 possess. 



CARNELIAN. 



Carnelian is red chalcedony. It may have every appearance of being uniformly coloured 

 or, on the other hand, the different layers of which it is built up may differ slightly, 

 though appreciably, in colour. Typical carnelian is of a deep flesh-red colour, hence 

 its name, but every shade of colour between this and pure white and yellow is represented. 

 It was mentioned above that white chalcedony is often termed white carnelian ; this term 

 is most correctly applied to chalcedony with a faintly reddish or yellowish tinge, every 

 possible shade between this and the deep flesh-red of typical carnelian being represented in 

 natural stones. In some cases portions of the stone are paler than others, and specimens 

 with a pale central portion passing gradually into a dark coloured exterior are not at all 

 uncommon. Fine dark stones of uniform colour and free from faults are described as 

 " carnelian de la vieille roche," or as " masculine carnelian." By transmitted light they 

 appear of a deep blood-red colour, and in reflected light of a blackish-red shade. They are 

 found in India, but are very rare, and on account of their great beauty are highly esteemed. 

 Stones of a pale red, or of a yellowish-red colour, are described as " feminine carnelian,'" 

 or simply as carnelian. Among several thousand stones there will be probably only a few 

 to which the term " masculine " may be be applied ; the rest will be yellowish, brownish, 

 greyish, or too pale in colour, patchy or disfigured by cracks and fissures. Compared with 

 opaque red jasper, which is often of a carnelian colour, all carnelians, whatever be their 

 colour, are strongly translucent. 



The colour of carnelian is not due to the presence of organic compounds, as was once 

 supposed, but to that of compounds of iron, the red colour being due to ferric oxide, and 

 the yellow and brown to hydrated ferric oxide. Yellow or brown stones, when exposed to 



