HARDNESS 29 



as it is more or less characteristic, it must be taken into account in the identification 

 of rough stones. 



The fractured surface frequently has the rounded form of a molluscan shell, and 

 the two surfaces, respectively convex and concave, which fit together, both exhibit 

 regular, circular ridges and grooves concentric about the point where the specimen 

 received the blow which caused the fracture. Since these circular markings resemble the 

 lines of growth on a molluscan shell, this type of fracture is known as a conchoidal 

 fracture. Perfect conchoidal fracture is shown by artificial glass and also by the natural 

 volcanic glass, obsidian. The surfaces of the conchoidal fracture may vary in extent, 

 and also in the degree of curvature, being at times almost plane, hence such fractures 

 may be distinguished as flat- or deep-conchoidal, and large- or small-conchoidal. The 

 last, or sub-conchoidal fracture, merges into what is known as uneven fracture. As 

 mentioned above, the fractured surface may approximate to a plane surface, never, 

 however, being truly plane ; this is known as even fracture, and is well shown by jasper ; 

 it merges into the flat- and large-conchoidal types. 



Sometimes the fragment separated from a specimen by a blow from a hammer shows on 

 the fractured surface loosely attached splinters, lighter in colour than the main mass of the 

 stone. This class of fracture is said to be splintery, and is excellently illustrated by chryso- 

 prase. Naturally there may exist every gradation between a typical splintery fracture and 

 a smooth fracture. 



Precious stones are frequently penetrated by cracks which are the forerunners of 

 fractures. Such cracks considerably lessen the transparency and beauty, and consequently, 

 the value of a stone ; specimens showing such flaws are avoided by gem-dealers. Though of 

 rare occurrence in many stones, in others, e.g., emerald, they are often present in great 

 numbers. 



These internal cracks due to fracture resemble the cracks shown by stones possessing a 

 cleavage in that they often exhibit iridescent colours ; as, for example, is sometimes seen 

 in rock-crystal. Here, however, the ci-acks, and the bands of colour to which they give rise, 

 are more or less markedly curved, and are thus quite distinct from the plane surfaces of 

 cleavage cracks. 



C. HARDNESS. 



For a mineral which is to be used as a gem, an important and, indeed, indispensable 

 property is that of hardness. By the hai-dness of a mineral is understood the resistance 

 which it offers to being scratched or marked by another body. The greater the resistance, 

 the harder the mineral. Only the harder stones, when used as gems, are capable of 

 preserving unimpaired their transparency, lustre, and play of colours. Softer stones when 

 newly cut may display all these qualities, but in use they soon become scratched on the 

 surface, which detracts considerably from their beauty ; a single scratch on the back of 

 a transparent stone, that is on the side away from the observer, is many times reflected, and 

 thus the bad effect is multiplied. The beauty of opaque stones is also greatly marred by 

 scratches, but those on the front side of the stone only will be observable in this case. 



The degi'ee of hardness is of considerable importance in identifying and distinguishing 

 precious stones, and is a character of which the dealer in gems should make frequent use. 

 Hence the necessity for acquiring a knowledge of the different degrees of hardness possessed 

 by different stones. 



The method of testing the relative hardness of two stones is simplicity itself. A sharp 

 corner of the one is rubbed with a certain pressure across a smooth surface of the other ; 



