THE FELSPAR GROUP (MOON-STONE) 429 



hardness of quartz is 7, that of avanturine-felspar is only 6, so that the latter is readily 

 scratched by quartz. Artificial avanturine-glass made in imitation of sun-stone will be 

 considered under avanturine-quartz. 



MOON-STONE. 



The name moon-stone is applied to colourless, very translucent, or almost perfectly 

 transparent felspar, which in a certain direction reflects a bluish, milky light, that has been 

 •compared to the light of the moon. This peculiar feature, like the metallic sheen, 

 characteristic of sun-stone, is not confined to one member of the felspar group, but is 

 ■exhibited by isolated examples of all the different varieties. It is shown to perfection in 

 orthoclase, and for this reason moon-stone is frequently referred to as a variety of adularia, 

 and the appearance characteristic of this stone as adularescence. It is by no means correct, 

 however, to suppose that this feature is peculiar to adularia, for the same reflection of milky 

 light is to be seen, though rarely, in the colourless and transparent soda-felspar, which is 

 known to mineralogists as albite ; moreover, the same is true of felspars having the chemical 

 •composition of oligoclase, which have been dealt with already under sun-stone. These 

 adularescent felspars showing a reflection of soft, milky light, are variously referred to as 

 girasol, fish-eye, wolf's-eye, " Ceylonese opal " or " water-opal." 



Adularia is the felspar which most frequently shows the characteristics of moon-stone. 

 It is a colourless and translucent to transparent variety of pure potash-felspar, for which 

 the mineralogical term is orthoclase, and its chemical composition is represented by the 

 formula K20.Al203.6Si02. It frequently occurs as fine crystals of large size and very simple 

 form, such as are represented in Figs. 80a and b ; it also takes the form of complex twin- 

 crystals, as in Fig. 80c, which represents four individuals united together in twin position. 



The specific gravity is 2'55, the same as for all orthoclase. The milky sheen 

 ■characteristic of adularia is not exhibited over the whole surface of the stone, but only over 

 a certain portion near a face which truncates the front and back edges of the prism (Figs. 80a 

 ■and b), and in massive specimens over a corresponding part. The milky sheen is only 

 visible when this part of the stone is in a certain position relative to the source of light and 

 the eye of the observer. If the stone be turned out of this position the reflection of milky 

 light ceases, but the sheen reappears when the stone is replaced in its original position. In 

 Fig. 4 of Plate XVI. an attempt is made to reproduce the appearance characteristic of an 

 irregular cleavage fragment of moon-stone. 



Only specimens which show this sheen in a typical manner are cut as gems. To display the 

 beauty of this stone to the best possible advantage, it is cut with a convex, polished surface 

 of moderately strong curvature about the face which exhibits the sheen. The stone is very 

 effective in cut and polished spheres, which may be strung together and worn as beads or for 

 other ornamental purposes, when they have a certain resemblance to white pearls. Moon-stone 

 is always cut either en cabochon or with a plane surface, never with facets. If the plane face 

 be cut so that it is suita,bly orientated with respect to the crystal, there will be a milky sheen 

 uniformly distributed over its surface ; but if, on the other hand, the stone be cut with a 

 convex surface, there will appear a band of bluish, milky light, crossing a colourless and 

 almost transparent background (Plate XVI., Fig. 5). The area occupied by the band of 

 light appears almost wholly lacking in transparency, while the area surrounding the band 

 exhibits no milky sheen. There is no very sharp line of demarcation between the chatoyant 

 band of light and the surrounding portions of the stone, but it is sufficiently abrupt to form 

 an effective contrast. The more convex the polished surface of the stone the narrower and 



