THE DIAMOND AND OTHER PRECIOUS STONES. 355 



for this that tlie coating sliould bo very tliiii so as merely to dull tlic 

 glass, and that the finger sliould be moved directly across — for cxnniple, 

 from right to left, or from above downward ; then looldng through it at 

 a lighted candle, there will be seen a band of white light crossing the 

 direction of the lines of tarnishing. If the same operation is performed 

 in two directions on opposite sides of the glass, then a ibur-limbed cross 

 will be formed by the two luminous bands which cross each other be- 

 fore the eye. 



Ceylon produces a greenish stone traversed by filaments of white 

 amianthus, which is called the cafs-ct/e, and which is usually cut spheri- 

 cally and quite prominent. We see in it a floating band, which comes 

 from the play of light on the lines of amianthus within it. In general 

 with these curious accidents of light exhibited by exceptional stones, 

 the color of the starry radiance should contrast as much as possible 

 with the tone of the stone itself. In simply scratching crossed lines on 

 a beautiful carnelian, I have succeeded in producing a white cross on a 

 red ground. In minerals this starry quality is very valuable, because 

 it reveals the primitive form of the substance in which it is found, and 

 I repeat that, by looking through a stone suitabl}^ cut, we find these 

 luminous transverse bands in a great variety of crystallized minerals. 



There is a very hard dust employed in the arts, called emery, a pow- 

 der used in rubbing or grinding down bodies with hard surfaces. This 

 substance is a species of corindon or sapphire, containing a tolerably 

 large proportion of iron, which has been substituted for the aluminium 

 at the time of the formation of the stone. This substitution is quite 

 common in chemistry and mineralogy. It is believed that the Chinese 

 succeed, by patience, in cutting diamonds with emery. This must be 

 very slow work, because the stone of w^hich emery is composed is ^'cry 

 much softer than the diamond ; it is like sharpening steel by rubbing it 

 on paper or linen. However, if patience can work miracles, it is doubt- 

 less reserved for the Chinese to accomplish this result. 



We shall place after the sapphire, the oped, which comes from Hun- 

 gary and Mexico. The Hungarian opals are much the superior, and 

 have not the disadvantage of deteriorating with time. Some years ago 

 the opal was higher in price than the sapphire ; but increase in value 

 inducing a more active working of the mines, the price of opals, beauti- 

 ful as they are, fell to what we find it at present. For the perfection of 

 an opal, it should exhibit all the colors of the solar spe(itrum, disposed 

 in small spaces, neither too large nor too small, and with no color pre- 

 dominating. The opal is sometimes called the harlequin, in allusion to 

 the great variety of colors which it displays. The substance of the opal 

 is of a milky hue and of a pale greenish thit. This milkincss is gener- 

 ally known by the term opalescence. It is the color of water in which 

 a little soap has been dissolved. In order to explain the brilliant colors 

 of the opal, we may imagine hv the stone a great number of isolated fis- 

 sures, of variable width, but always very narrow. Each fissure, accord 



