34 GENERAL CHARACTERS OF PRECIOUS STONES 



D. OPTICAL CHARACTERS. 



Those qualities of precious stones which depend on their behaviour towards light are 

 known as optical characters, and are of special interest and importance. The transparency 

 and lustre, the colour and play of colours of a stone, depend largely on its optical characters. 

 Moreover these qualities furnish a means whereby the stone may be easily determined, and 

 with no risk of injury such as accompanies the testing of its hardness. It is important, 

 therefore, to be acquainted, at least to a certain extent, with the laws of optics, and with 

 some of the instruments used in the investigation of optical phenomena ; these are dealt with 

 below as fully as space allows. 



1. TUANSI'AIIENCY. 



The majority of precious stones are transparent, but in the uncut condition the free 

 passage of light through the stone is often obstructed by rough and uneven faces. When 

 such faces are removed by cutting, and the cut surfaces polished, an apparently cloudy 

 specimen often becomes beautifully clear and transparent. The transparency of such costly 

 stones as diamond, ruby, and sapphire, is often very perfect, while the same property exists 

 in less costly stones, such as rock-crystal and amethyst. The greater the transparency of a 

 specimen of any given precious stone the more highly is it prized. The only jewels of the 

 first rank which are not transparent are the noble opal and the turquoise ; in stones of less 

 value, such as agate, chrysoprase, malachite, and others, this is more frequently the case. 



Transparent bodies allow the free passage of light through their substance ; an object 

 viewed through a perfectly transparent substance will have no blurred edges, but will present 

 a clear and sharp outline. A stone which combines complete absence of colour with perfect 

 transparency, as in diamond and rock-crystal, would be described as water-clear or limpid. 

 A perfectly water-clear stone with the highest degree of transparency, and free from any 

 trace of colour, is known to jewellers as a stone of the first or purest water, and stones of this 

 high quality are especially prized. Should the stone show a very slight cloudiness or tinge 

 of colour, scarcely noticeable perhaps to the unpractised eye, it is known as a stone of the 

 second water ; similarly a stone which shows a further departure from the standard of per- 

 fection in these qualities is known as a stone of the third water. This subject will be again 

 reverted to under the special description of diamond. 



A substance which in a mass of some thickness allows a large proportion, but not all, of 

 the light emanating from any source to pass through it, is known as a semi-transparent 

 substance. Any object, for example a flame, viewed through such a substance will not be 

 seen distinctly, but will be blurred in outline. A substance through which some of the light 

 of a flame can pass, but through which it is impossible to see even a blurred outline of the 

 flame, is described as being translucent. As an example of a semi-transparent stone chalce- 

 dony may be mentioned, while opal is an example of a translucent stone. In some cases 

 light can only pass through a very thin splinter or a sharp edge of a broken stone ; such 

 stones are translucent only at their edges, being quite opaque in mass. A chrysoprase held 

 in front of a light will show a dark centre surrounded by a lighter border. Clpaque stones, 

 even when thin, completely cut off' all light, hence when held before a light they present a 

 uniformly dark outline with no lighter border. Opaque stones then, for example haema- 

 tite, owe their beauty not to their transparency, but to the fineness of their lustre and 

 colour. 



Different specimens of the same kind of stone vary greatly in transparency, and 

 consequently in value ; while one specimen may be perfectly transparent, another may be so 



