CLASSIFICATION 107 



manner ; characters which are liable to vary in different specimens, such, for instance, as 

 colour, ai-e regarded as non-essential. It is far otherwise, however, in the case of precious 

 stones ; here the application of a particular specimen depends largely on its colour, hence 

 this character plays an important part in the grouping of precious stones according to the 

 second method of classification, while chai'acters, such as chemical composition and 

 crystalline form, having but slight influence on the application of a stone for ornamental 

 purposes, are much less relied upon. 



Owing to these differences in the principles of classification, it is easy to understand 

 that many stones which may be brought together in a mineralogical classification, on account 

 of their similar chemical composition and crystalline form, under the same species, and may 

 be known by the same name, may, in the artificial system of classification, be divided among 

 several groups and be known by different names on account of differences in colour. On 

 the other hand, stones of the same colour, which a jeweller may consider of the same kind, 

 and to which he may apply but one name, or at least one with a qualifying prefix signifying 

 small differences of colour or hardness, may by a scientific mineralogist be grouped under 

 different species according to their chemical and crystallographic differences and be recog- 

 nised by different names. 



A good example of the first case is afforded by the mineral sfiecies corundum. 

 Mineralogists include in this species all those stones which are composed of pure alumina 

 and which crystallise in the hexagonal system. The stones of this species are all of the 

 same hardness (H = 9) and specific gravity (sp. gr. = 4), while other physical characters are 

 equally constant. It is therefore in accordance with the principles of scientific classification 

 that such stones should be grouped in the same species and be known by the same name. 

 Different specimens of this species of mineral, however, may differ widely in colour ; red, 

 blue, yellow, green, yellowish-green, greenish-blue, yellowish-red, violet, and colourless 

 specimens having all been found. All the colour-varieties mentioned above are not of equal 

 importance for purposes of ornament, but they are considered by the jeweller, in spite 

 of their mineralogical identity, as distinct and separate stones, and as such are distinguished 

 by special names ; these names are given below in the order in which the colour-varieties 

 were mentioned above : ruby, sapphire, " oriental topaz," '' oriental emerald," " oriental 

 chrysolite," "oriental aquamarine," " oriental hyacinth," "oriental amethyst," white sap- 

 phire (leuco-sapphire). The mineral beryl is another case in point ; the mineralogist 

 includes the deep green, bluish-green, greenish-blue, and yellow specimens in the same species, 

 to which he gives the name beryl, since they all agree in chemical composition and crystalline 

 form, and differ only in colour ; the jeweller, on the contrary, refers to the deep green 

 variety as emerald, to the greenish-blue and bluish-green varieties as aquamarine and to the 

 yellow varieties as beryl. 



Another example of the method of classification adopted by jewellers may be given. 

 All light greenish-yellow to yellowish-green transparent stones, whatever may be their 

 chemical composition and crystalline form, are referred to by jewellers as chrysolite. Thus 

 this name comes to include such essentially dissimilar minerals as olivine, chrysoberyl, 

 idocrase, corundum, and the peculiar moldavite or bottle-stone. To distinguish these one 

 from another, such descriptive terms as olivine-chrysolite, opalescent chrysolite for 

 chrysoberyl, "oriental chrysolite" for yellowish-green corundum, &c., are used. The 

 original signification of the prefix "oriental " has already been explained. 



Minerals used as precious stones may be classified into groups according to various 

 systems : thus their position may be decided by the feature on which their beauty depends ; 

 bv their essential mineralogical characters ; by the frequency of their occurrence in nature ; 



