INTRODUCTION 



Cektain minerals occumng in the earth's crust are distinguished by special beauty, and 

 have therefore been used since the earliest times for personal ornament and for decorative 

 purposes generally. The beauty of such minerals depends upon their transparency, lustre, 

 or colour, or in some cases upon a play of colour, due to the modification of rays of light 

 reflected from the surface or transmitted through the stone. This beauty is not manifested 

 to its fullest extent until the stone is cut and polished, when these features become more 

 conspicuous. In some cases, for example in the rarely occurring diamonds of a beautiful 

 red or blue colour, all these features are present in the same stone ; in others, as in the ruby, 

 the play of colour is absent, and the effect of the stone depends upon its transparency, 

 lustre, and colour. In a stone such as precious opal, the beauty of the mineral depends 

 solely upon a play of colour, which is independent of the colour of the stone itself. Opaque 

 minerals, like turquoise, with but little lustre, owe their beauty to their fine colour; finally, 

 colour may be completely absent, and the beauty of the stone due to its transparency, lustre, 

 and play of colour, as in the purest colourless diamonds. 



Those minerals which, through the possession of some or all of the features enumerated 

 above, lay claim to beauty of appearance are not all equally suitable for gems. Besides the 

 possession of undeniable beauty, which for its use as a gem-stone is naturally a sine qud non, 

 & mineral must also possess a certain degree of hardness, for otherwise a very small amount 

 of wear will suffice to dim its beauty. Even should it successfully resist the effect of contact 

 with the moisture of the skin, a comparatively soft stone will succumb to the action of grit 

 and dust, which for the most part consist of particles of the mineral quartz. It is desirable, 

 therefore, that all minerals used for personal ornament should possess at least the hardness of 

 ■quartz, and a still greater hardness, the so-called gem-hardness, is an advantage. Some 

 discretion should be exercised in the setting of stones of different degrees of hardness ; thus 

 Si comparatively soft stone may be quite suitable for the ornamentation of a brooch, but 

 should not be set in a ring, where it is likely to get much hard wear. In the same way 

 minerals, which, however beautiful in the fresh condition, lose their beauty on exposure to 

 the atmosphere are unsuitable for use as gems. 



We have now shown that only those minerals, which combine beauty of appearance 

 with considerable hardness, and a power of resisting external influences, are suitable for use 

 as ornaments. The mineral substances so distinguished are known as precious stones or 

 gems. 



All the essential characters of a gem occur together in but few minerals, so that the 

 number of precious stones is small compared with that of all the mineral species known. 

 Moreover, the minerals distinguished by the possession of the characters of a gem occur for 

 the most part, at least in pieces of good size and quality, in very sparing amount, so that 



