406 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



The dichroism of olivine is always feeble ; the two images seen in the dichroscope 

 vary in colour between yellowish oil-green and pure grass-green with no appreciable yellow 

 shade. 



The other characters of this mineral, not affecting the appearance ot the gem, but ot 

 use in the recognition of uncut stones, may be mentioned, namely, the fusibility before the 

 blowpipe and the behaviour of the stone towards acids. Only olivine very rich in iron is fusible 

 before the blowpipe ; this particular variety, which is never used as a gem, can be fused, but 

 always with difficulty. All varieties of olivine when reduced to a fine powder are quickly 

 decomposed, especially when warmed, by hydrochloric or by sulphuric acid, with separation 

 of gelatinous sihca. This character is utilised, probably unintentionally, in the process of 

 polishing chrysolite, the final polish being given by the use of sulphuric acid instead of 

 water. 



The table-stone and the step-cut in their various modifications are the forms usually 

 employed in the cutting of chrysolite ; one such form is shown in Fig. 12 of Plate XIV. 

 Brilliant and rose forms are also sometimes employed. The table facet of table-stones and 

 step-cuts is not infrequently cut with a curved surface, the result being a transition form 

 between a faceted stone and the ordinary curved (cabochon) form which is sometimes used 

 for chrysolite. The colour and lustre of olivine are often improved by the use of a gold 

 foil, or in the case of very pale stones of a green foil. 



Olivine is widely distributed throughout the rocks of the earth's crust. It is a 

 constituent of basalt, and occurs most frequently in irregular grains or in large granular 

 aggregates the size of a man's fist or head, or even larger, and consisting of small irregular 

 grains of olivine intermixed with a few fragments of other minerals ; it is only rarely found 

 in sharply defined crystals. Olivine occurs also in other igneous rocks, such as diabases 

 and gabbros ; and rock-masses of large size are to be met with which consist wholly or 

 largely of pure olivine. The mineral again is sometimes found interlaminated in gneiss 

 or crystalline schists. Grains of olivine are sometimes to be found in the weathered 

 debris of such rocks, but the mineral is usually the first to become altered by exposure 

 to weather. Finally, it should be mentioned that olivine is an essential constituent of 

 many meteorites. 



The olivine found under the conditions mentioned above is scarcely ever suitable 

 cutting as gems. That which occurs in diabase and gabbro is impure and opaque ; the 

 material found in basalt and similar rocks, as, for example, that from Vesuvius, is frequently 

 pure and transparent, but is almost always in grains of quite small size. The component 

 grains of the larger aggregates also are nearly always of small size ; only rarely, as in the 

 basalt of Mount Kosakow, near Semil on the Iser, in northern Bohemia, are they found of 

 any considerable size. At this locality there sometimes occur transparent pieces the size 

 of a hazel-nut and of a fine green colour from which gems can be cut. The olivine found 

 in meteorites is also sometimes transparent and of a fine colour, but the grains or crystals 

 in which it occurs are always of small size ; in only one or two extra-terrestrial bodies have 

 fragments of transparent olivine been found sufficiently large to yield one-carat stones, and 

 these are in very truth celestial gems. 



The source of the chrysolite which is used in the trade for cutting as gems is 

 somewhat obscure. It is probably identical with that of the transparent, finely coloured 

 pebbles not infrequently seen in mineral collections. These pebbles are similar in all 

 respects to cut chrysolites ; the largest are about the size of a walnut and they have 

 obviously been collected in a river sand or gravel, the exact locality of which is, however, 

 unknown. 



