58 GENERAL CHARACTERS OF PRECIOUS STONES 



chemical relation between the colouring-matter and the ground-substance is possible, are 

 clear and transparent. 



The apparent change of colour shown by many precious stones when exposed to 

 diiFerent kinds of illumination is worthy of remark. In most cases the colour seen in clear 

 day-light is the most beautiful, the appearance by artificial light being less pleasing. Thus 

 amethyst by day-light is of a beautiful purple colour, but in candle-light it appears dull 

 grey. Purple corundum or " oriental amethyst," on the contrary, shows its fine colour as 

 well by candle-light as by day-light. Specially peculiar in this respect is the variety of 

 chrysoberyl, known as alexandrite, which, as we shall see later on, is green in day-light and 

 red in candle-light. Yellow diamonds retain their colour in the electric light, but appear 

 colourless in candle-light. Many other stones afford similar examples of a change in colour 

 accompanied by a loss of beauty in artificial light, a property which naturally diminishes 

 the value they might otherwise possess. 



The possibility of temporarily masking the colour of yellowish diamonds has, in recent 

 years, frequently led to fraud. Since the discovery of the South African mines, yellowish 

 diamonds are fairly abundant, and therefore comparatively cheap, while perfectly colourless 

 stones command a high price. By giving these yellowish stones a very thin coating of some 

 blue colouring-matter, they can be made to appear colourless, the mixture of blue and 

 yellow light rays producing on the eye the effect of white light. As soon, however, as the 

 blue coating is worn off the fraud becomes apparent. 



Not only an apparent, but an actual change of colour, may be experienced by some 

 precious stones. As a rule, the colours of precious stones are extremely lasting, only dis- 

 appearing with the destruction of the stone itself, this being, for example, the case with the 

 yellow diamond, the ruby, emerald, and others. The colour of other stones, however, is less 

 constant, and may be completely destroyed, the substance of the stone undergoing no change 

 in the process. The colouring of such a stone will frequently disappear when the stone is 

 raised to a red heat or even less ; this will invariably happen if the colouring-matter is 

 organic in nature, since it will be decomposed at such a temperature. Brown smoky-quartz 

 and reddish-yellow hyacinth behave in this way, becoming completely colourless when heated 

 to redness. Other stones on being heated experience not a loss but a change of colour ; 

 thus the violet amethyst becomes yellow, and the dark yellow topaz becomes rose-red in 

 colour. These particular changes in colour are sometimes brought about intentionally in 

 order to obtain yellow quartz (" burnt amethyst ") and rose-red topaz (rose topaz), both of 

 which are used as cut stones, but occur in nature to only a small extent. 



Many stones show characteristic changes in colour during the progress of a rise and fall 

 in temperature. Thus the red ruby, at a high temperature, is colourless ; on cooling it first 

 becomes green, after which it gradually assumes its original fine red colour. The red spinel 

 behaves somewhat differently under similar conditions ; at a high temperature it becomes 

 colourless, and on cooling it regains its original colour, so far resembling the behaviour of 

 the ruby, but at the intermediate temperature it assumes not a green but a violet tint. A 

 high temperature is not invariably necessary to effect a change of colour in precious stones ; 

 some stones are so sensitive that their colour fades or disappears merely on exposure to light 

 and air. Certain topazes behave in this way, and after a few months exposure will be 

 recognisably paler in colour ; the same phenomenon may be observed in green chrysoprase 

 and in rose-quartz, as well as in some blue turquoises, the colour of the latter of which may 

 gradually change to green. Obviously the value of such stones will be considerably 

 diminished, since it is difficult, if not impossible, to avoid a rapid loss of colour, and there- 

 fore of beauty, when they are used under ordinary conditions. Colour lost in this way may 



