512 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



shades of grey, blue, yellow, or brown, while the colours of others may be pronounced shades 

 of yellow, red, brown, or grey, green and blue being always rare. Each single layer is 

 usually uniformly coloured throughout its whole extent, and can be identified with one or 

 other of the colour-varieties of chalcedony already considered. Thus the pale-coloured 

 layers agree in character with common chalcedony, the red with carnelian, the brown with 

 sard, and so on. According to the predominant colour of a stone it may be distinguished 

 as chalcedony-agate, carnelian-agate, &c. ; and here also may be mentioned agate-jasper 

 (jasp-agate) already described, in which layers of translucent chalcedony alternate with 

 bands of opaque j asper. 



The beauty of agate and its application to ornamental purposes depend upon the 

 contrast in colour and translucency between the bands of which it is built up. Finely 

 marked and very translucent agates are sometimes distinguished by the prefix " oriental," 

 stones less perfect in these respects being referred to as " occidental" agates. The majority 

 of agates as they occur in nature are light coloured, and but few show strongly marked 

 colour contrasts, such as deep shades of red, yellow, or brown. These stones, however, like 

 ordinary chalcedony, can be artificially coloured, as we shall see later on when the methods 

 of working agate are under consideration. 



The two figui-es of Plate XIX. illustrate the varied courses taken by the bands shown 

 on a cut and polished surface of agate. According to the direction and arrangement of the 

 bands, different kinds of agate are distinguished. 



In riband-agate the different layers are parallel to each other, their surfaces being 

 plane or uniformly curved, without indentations or prominences. A surface cut perpendicular 

 to the layers will show straight or curved bandings. A riband-agate in which milk-white 

 cloudy bands alternate with bands of another colour, the two sets being sharply marked off' 

 from each other, is known by the general term onyx. According to the shade of the 

 coloured bands several sub-varieties are recognised. Thus, the term onyx in its more 

 restricted sense is applied to agate in which the second set of bands are black ; when these 

 coloured bands are one of the pale shades characteristic of common chalcedony, the stone is 

 described as chalcedony-onyx, when they are red as carnelian-onyx, and when brown as sard- 

 onyx. From the point of view of the working of agate, the different varieties of onyx have 

 a definite importance, and will be referred to again. In a modification of riband-agate, 

 known as ring-agate, the difl^erently coloured bands are disposed in. concentric circles. A 

 stone with a dark coloured central spot surrounded by a series of concentric rings has a 

 certain resemblance to an eye, and is therefore referred to as an eye-agate. A kind of ring- 

 agate can be artificially produced from ordinary agate or chalcedony : the point of a steel 

 rod is placed on a cut surface of agate and is then smartly struck with a hammer. There is 

 then formed around the point a system of concentric rings, which give the stone a very 

 pretty appearance. 



When the banding is disposed so as to form re-entrant and salient angles, its outline 

 being the same as that of the plan of a bastion or fortress, the agate is cs^eA Jbrtiflcat'ion- 

 agate. The markings of landscape- agate suggest the outlines of a landscape picture, and 

 when the banding recalls the outlines of a ruin, as is the case in brecciated-agate, to be 

 described below, the stone is described as ruin-agate. In cloud-agate cloudy patches 

 contrast with a more translucent background ; star-agate shows star-shaped figures ; shell- 

 agate or coral-agate resembles in appearance fossilised shells and corals, the petrification of 

 these objects in agate actually at times taking place. A number of other names, most of 

 which are descriptive of the sub- variety to which they are applied, are recognised, but need 

 not be enumerated here. 



