404 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



OLIVINE. 



The olivine group embraces a large number of minerals, of which, however, only the one 

 from which the group takes its name has any application as a gem. Pi-ecious olivine is 

 known as chrysolite and to French jewellers as peridot. The name chrysolite is given to this 

 stone on account of its fine yellowish-green colour, but the term in its original meaning is 

 more correctly applied to stones of a pronounced golden-yellow, such, for example, as yellow 

 topaz, which is referred to by Pliny and other ancient writers under this name. The name 

 olivine is also descriptive of the colour of the stone, which is always a green showing shades 

 of yellow and brown like that of the fruit of the olive-tree. No other colour is ever shown 

 by chrysolite of gem-quality, so that this stone differs from many of those hitherto 

 considered, such as diamond, corundum, topaz, in which the range of colours is very large, 

 and in this respect it is more like turquoise. 



The chemical composition of chrysolite, and indeed of all the minerals of the olivine 

 group, is comparatively simple. Common olivine, and the gem-variety chrysolite, is a 

 silicate of magnesium having the formula 2MgO.Si02 ; a portion of the magnesia is, 

 however, always replaced by a corresponding amount of ferrous oxide. The following is 

 the result of an analysis made by Stromeyer of fine, transparent, yellowish-green chrysolite of 

 gem-quality from the Orient. 



Per cent. 



Silica (SiOj) . 39-73 



Magnesia (MgO) . . 50-15 



Ferrous oxide (FeO) . ... 9-19 



Manganous oxide, nickelous oxide, and alumina were also present in minute amounts, 

 but the presence of these constituents need not be further considered here. The amount of 

 iron present varies to a certain extent, and it is on this variation that the differences 

 between the shade of different specimens depend. The colour of chrysolite, as of green 

 bottle-glass, is due solely to the presence of a small amount of ferrous oxide ; when this is 

 present in larger amount the stone is darker in colour, and vice versd. In connection with 

 this subject may be mentioned the fact that a silicate of magnesium known as forsterite, a 

 mineral of the olivine group, contains no iron and is perfectly colourless. 



Olivine sometimes occurs as distinct crystals, which belong to the rhombic system. A 

 form not infrequently taken by transparent chrysolite of gem-quality is that shown in 

 Fig. 71 . Here the faces of the rhombic prism are inclined to each other at an angle of 

 130° 3', the side edges are truncated by a pair of small faces of the brachy-pinacoid, and 

 the front and back edges by a pair of large faces of the macro-pinacoid. The combination 

 of these three sets of faces gives an eight-sided prism flattened in the direction of the large 

 pair of faces of the micro-pinacoid. The faces of the micro-pinacoid and of the prism are 

 striated in the direction of their length, that is to say, parallel to the edges of the prism. 

 Above and below the prism faces are triangular faces of a rhombic octahedron or pyramid, 

 and above and below the pinacoid faces there are rectangular faces of a macro-dome and 

 a brachy-dome, the former being represented in the figure as the larger. Finally, the 

 crystal is bounded above and below by a pair of narrow rectangular faces or basal 

 planes, which truncate the two sets of dome-faces and the pyramid. Other forms are 



