OCCURRENCE 69 



surface. Some of the constituents of the rock are dissolved in water and carried away and 

 thus the cohesion of the mass is destroyed. The more or less loose, clayey, or sandy residue 

 is the weathered product, and this will contain the precious stones which were present in 

 the original rock, since, as a rule, they are unattacked by weathering agencies. The precious 

 stones will be present in the weathered product in relatively greater numbers than in the 

 original mother-rock. 



It will be readily understood that it is more profitable to work weathered mateiial than 

 the unaltered primary rocks for precious stones, for not only is the former relatively 

 richer in gem-stones than the latter, but it allows of the stones being easily separated or 

 washed out. The extraction of a gem-stone from solid rock involves much labour and 

 patience, and, even when every care is taken, may result in serious damage to the stone. 



The loose, incoherent material which results from the weathering of a rock, when it 

 contains a mineral worth extracting for technical purposes, is known generally as a sand,eiiid 

 as a g-em-sand when it contains precious stones. It is in such sands which, wherever they 

 occur, cover the solid rocks and form the outer portion of the earth's crust, that the most 

 valuable precious stones are found, such, for instance, as diamond, ruby, and sapphire. They 

 are separated from these masses of detritus by the process of gem-washiiig, in which the 

 heavier stones and larger fragments remain behind, while the lighter clayey and sandy con- 

 stituents are washed away. 



When the weathered material has not been carried away by the various transporting 

 agencies, but remains near the parent rock, the precious stones and other minerals it contains 

 will preserve intact the sharp edges and the crystalline form they possessed when embedded 

 in the solid rock. Such cases, however, are rare ; much more frequently the whole i>f the 

 loose material is transported in streams and rivers, and is finally deposited in a lower part of 

 the valley, far away from its original resting-place. In such river' sands and gravels, which 

 are known as alluvial deposits, the mineral fragments, and even the precious stones, in 

 spite of their hardness, become so rubbed by mutual friction during their travels that all 

 angularities are lost, and they present the appearance of smooth, rounded pebbles or grains. 



The presence or absence of this smooth water-worn appearance in the mineral fragments 

 of rock detritus is conclusive proof in the one case that water has been the transporting 

 agency, and in the other that it has not. The greater the hardness of the precious stone 

 transported in gravels by water, the less will be the rounding it undergoes; even diamond, 

 the hardest of materials, may show traces of rounding if the action of other softer stones is 

 only continued long enough. 



The precious stones, found in such water-worn materials, are frequently superior to 

 specimens which have not been subjected to the action of running water, and are still to be 

 found in their parent-rock. Such stones are frequently traversed by fissures, often scarcely 

 visible, but enough to make them unfit for use as gems, since, as has been mentioned before, 

 they have a tendency to break along these fissures. Precious stones which have been rolled 

 about and ground together in the bed of a river during long ages have undergone a fairly 

 se\'ere trial ; any which have a tendency to fragment will be reduced to splinters at an early 

 stage of their journey ; those, on the other hand, which survive may be considered to have 

 proved their durability. 



As regards the geographical distribution of precious stones, it may be mentioned 

 that in former times the most valuable came from India and other parts of the " Orient." 

 It was therefore believed in the Middle Ages that the glowing sun of tropical countries was 

 essential to the development of those qualities in precious stones which are so highly prized, 

 and that specimens from colder countries were deficient in these qualities. Every good stone, 



