452 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



and special care must be taken during the operation of cutting to avoid the development of 

 such fissures and cracks. 



The coppery sheen of hypersthene is exhibited only on that face parallel to which the 

 crystalline enclosures are arranged, that is to say, only on the plane of separation ; no trace 

 of it is to be seen on other faces, and, moreover, when there are no enclosures the sheen is 

 completely absent. It follows, therefore, that the minute enclosures embedded in the 

 substance of hypersthene are not only the cause of the separation plane, but also of the 

 coppery sheen characteristic of the stone. The beauty of hypersthene is best displayed by 

 cutting a stone with a slightly convex surface and with the circular or oval base parallel to 

 the plane of separation of the mineral. On polishing this surface it acquires a fine lustre 

 and its sheen is also thereby intensified. If the polished surface is flat the metallic sheen is 

 exhibited uniformly over the whole area, but when convex, the sheen is confined to that 

 portion of the surface which is directed towards the source of light, and is then stronger and 

 more intense. The effect of the sheen is diminished or entirely destroyed if the stone is not 

 cut exactly in the manner described ; and the effect of facets is rather to detract from the 

 appearance of a cut stone. 



The body-colour of hypersthene is of a dingy brownish-black, but it forms an effective 

 background for the strong coppery sheen. The mineral is perfectly opaque in mass but in 

 very thin sections is transparent ; the tabular inclusions described above are to be distinctly 

 seen when such sections are examined under the microscope. 



The specific gravity of hypersthene is 3'4. Its hardness is 6, so that the mineral is 

 scratched by quartz, but is itself capable of scratching glass. It is brittle and fuses 

 without difficu ty before the blowpipe, giving a black magnetic glass. It is not attacked by 

 acids. 



A few other minerals belonging to the pyroxene group, and more or less closely related 

 to hypersthene, sometimes contain enclosures aiTanged parallel to a certain face, which also 

 give rise to a metallic, or, in some cases, to a pearly, sheen. This is in all cases less beautiful 

 than the coppery sheen of hypersthene, but the stones are sometimes cut and polished 

 nevertheless. Their body-colour is not dark brown as in hypersthene, but some pale shade 

 of brown, grey, or green. These minerals, which are known by the names of bronzite, bastite, 

 and diallage, will be now briefly described and the features in which they differ from 

 hypersthene indicated. 



BRONZITE. 



Bronzite is in reality a hypersthene which contains rather less iron than ordinarily. 

 Owing to this difference in the chemical composition of the mineral, the specific gravity is 

 lower, being only about S% the colour is less dark, and the sheen paler and more 

 bronze-yellow in character, though still very strong and with a briUiant metallic lustre. 

 Bronzite sometimes shows indications of a fibrous sti-ucture, and when this is the case its 

 sheen has a certain resemblance to that of cafs-eye. It occurs in association with felspar, 

 and in masses sufficiently large for cutting, at Kupferberg in the Fichtelgebirge ; in serpentine 

 at Gulsen, near Kraubat, in Styria ; on the Seefeld-Alp in the Ultenthal, Tyrol, and at other 

 places. Bronzite is employed even less extensively than hypersthene. 



