430 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



better defined is the band of light ; this, however, is true only up to a certain point, for if the 

 band is too narrow it will appear dull. It is at no time very intense or strongly marked, 

 but has always more of the nature of a soft, pearly lustre comparable to the chatoyancy of 

 cymophane or cafs-eye, though the lustre of the band of light in the latter stone is more 

 silky than pearly in character. The effect of a cut moon-stone is heightened by mounting it 

 in a closed, black setting. 



The chatoyancy characteristic of moon-stone is exhibited to a variable extent in different, 

 specimens of adularia, but in the majority it is completely absent. Moreover, this feature 

 is often more prominent in one portion than in another of the same specimen, and in such 

 cases the inferior portions are cut away and the best used for cutting as gems. The more 

 pronounced the chatoyancy of a stone the greater its value ; one the size of a bean with a 

 fine milky sheen is worth from 25 to 40 shillings, and the value increases considerably with 

 the size. 



The milky sheen is probably due to the presence of microscopically small, colourless, 

 and brilliant crystal plates embedded in great numbers in the felspar, and arranged parallel 

 to the surface from which the reflection of milky light takes place. The presence of sucK 

 plates can be observed in all specimens of adularia which show a milky sheen, and the more 

 numerous are the plates the greater will be the prominence of this feature, which, however, 

 is completely absent when there are no enclosures to be seen. Chatoyant specimens of 

 adularia are never perfectly clear and transparent but always cloudy, though possibly only 

 slightly so, and this fact is also due to the same cause, namely, to the enclosure of small 

 foreign bodies. 



Splendid specimens of adularia are found at various places in the Tyrolese and Swiss. 

 Alps. The mineral occurs here in fine crystals, of the forms shown in Fig. 80 ; these,, 

 together with quartz and other minerals, are attached to the walls of crevices in gneiss and 

 other crystalline rocks. This is the most important locality for the mineral, but it is rare 

 for Alpine specimens to exhibit any chatoyancy at all, and still more rare to find the feature 

 sufficiently well marked to make the stone useful for ornamental purposes. 



The strongly chatoyant and beautiful moon-stone used for cutting is obtained almost 



exclusively from the island of Ceylon. Its mode of occurrence there differs from that 



which obtains in the Alps. Massive fragments of irregular shape, and as large, or larger 



than, a man's fist, are here embedded in a white kaolin-like clay, which has probably been 



derived from the weathering of a porphyritic igneous rock. In all probability the moon-stone 



was one of the original constituents of this rock and the only one to resist the action of 



weathering. It is found under these conditions among other places at Neura Ellia near 



which is a spot, to the south-east of Adam's Peak, which is marked on some maps as^ 



" Moonstone Plain." It is also of frequent occurrence nearer the centre of the island and 



indeed, is a constituent of many of the rocks of that neighbourhood. It has also been 



observed to occur in close association with spinel, and in this case probably originated, 



together with this mineral and possibly also with the ruby, in the same mother-rock 



namely, in a crystalline limestone. Ruby and spinel are found under the same conditions. 



at Mogok in Upper Burma, but here moon-stone is present in only small amount. By the 



weathering of the mother-rock the moon-stone is set free, carried down the rivers and 



streams, and, as rounded pebbles, finally collected, together with other precious stones, out 



of the gem-gravels of the island. It is said to be most abundant in gem-gravels at 



Belhngham, between the Point de Galle and Matura on the south coast of the island, but 



the greater part of the material imported into Europe from Ceylon for cutting is in the 



form of irregular masses, not in rounded pebbles, and has therefore been obtained from the 



