36 GENERAL CHARACTERS OE PRECIOUS STONES 



the " fire " of a precious stone is connected with its lustre, and this quahty only exists with a 

 specially high degree of lustre ; the term " fire " has, however, sometimes another meaning, 

 as we shall see further on under Optical Dispersion. Very few of the more valuable precious 

 stones are devoid, in the cut condition, of a brilliant lustre ; of these the most important is 

 turquoise, which, even after being polished to the fullest possible extent, shows a certain 

 dulness of surface. This is due to a certain extent to the softer character of the stone ; for 

 the harder precious stones, such as diamond and ruby, are susceptible of a higher degree 

 of polish than the softer stones, such as turquoise. 



Each kind of stone is capable of receiving a certain degree of polish depending on the 

 physical characters of its substance. And while there is for every stone a certain maximum 

 lustre, which cannot be exceeded, even with the most persistent polishing, this lustre may 

 often, from a variety of causes, fall below the maximum. 



Different precious stones are not only characterised by different degrees of intensity of 

 lustre, but also by different kinds of lustre ; and this in many cases enables one to 

 distinguish stones which are otherwise similar in appearance. Thus it would be possible 

 for the least practised eye to distinguish at a glance a genuine diamond from its imitation 

 in rock-crystal, simply by the difference in lustre. 



Degree of lustre and kind of lustre are both loosely referred to as lustre. Though it 

 has been attempted to give some idea of the different degrees of lustre existing in different 

 stones, yet no adequate conception of the different kinds of lustre can be derived from a 

 mere verbal description. It is far preferable to acquaint oneself with the different kinds of 

 lustre by actually comparing substances showing the different kinds. Thus a piece of 

 burnished metal, a sheet of glass, a polished diamond, the mother-of-pearl lining of a shell, 

 a layer of greasy oil, or a piece of satin, may each possess a high degree of lustre, the quality 

 or kind of which is however different in every case. The different kinds of lustre exhibited 

 by the substances just mentioned are taken as types ; and the different kinds of lustre 

 existing in minerals can be referred to one or other of these types. Thus we have metallic 

 lustre, glassy or vitreous lustre, adamantine (diamond) lustre, pearly lustre, greasy lustre, and 

 silky or satiny lustre ; while more minutely descriptive terms can be derived from these, as, 

 for example, metallic-adamantine, metallic-pearly, &c. 



In the description of a mineral it is important to be able to recognise and' name its 

 particular kind of lustre. As explained above, each kind differs in intensity, so that we may 

 have strong or feeble vitreous lustre, &c. Both degree and kind of lustre depend on the 

 properties of the particular substance and are therefore, under certain conditions, 

 characteristic of that substance. It is, however, to be noted that the kind of lustre may be 

 considerably modified by certain peculiarities of structure shown by a mineral, so that it si 

 not an invariable character of a mineral species. The kind of lustre depends not only on 

 light reflected from the surface of a stone, but also, in part, on light which has penetrated 

 into its substance and suffered some modification before again passing out. As examples of 

 this influence of structure on lustre may be cited the silky lustre of satin-spar, a finely 

 fibrous variety of calcite, and the greasy lustre of elaeolite, a variety of nepheline containing 

 vast numbers of microscopic enclosures; contrasted with, these is the vitreous lustre shown 

 by the more usual varieties of the minerals calcite and nepheline. 



Metallic lustre, being exhibited exclusively by perfectly opaque substances, is found 

 in only a few of the less important precious stones, as, for example, haematite. 



Vitreous lustre, on the other hand, is best shown by perfectly transparent minerals, 

 and is of very frequent occurrence ; it is present to a more or less marked degree in the 

 majority of transparent precious stones, such as rock-crystal, topaz, ruby, sapphire, emerald. 



