CORDIERITE 409 



more lustrous, but is always far inferior to sapphire in this respect. The refraction is low, 

 ■and the double refraction is feebler than in any other precious stone. 



The range of colour shown by cordierite is rather extensive. Colourless, yellow, green, 

 and brown stones are met with, but the most commonly occurring are of a moderately dark 

 blue colour sometimes tinged with violet; only these latter are cut as gems. The most 

 transparent and the most finely coloured stones are cut from pebbles found in Ceylon, the 

 chemical composition of which has been given above. The bulk of the material used for 

 cutting is derived from this source, that from other localities being, as a rule, cloudy and 

 poor in colour. The colour of these pebbles varies between sky-blue and indigo-blue, the 

 paler being sometimes distinguished as water-sapphire and the darker as lynx-sapphire. 



The most salient feature of cordierite is its dichroism, that is to sav, the difference in 

 colour shown by the same stone when viewed in different directions. The phenomenon is 

 more marked in this than in most other minerals, hence its name of dichroite. The 

 maximum differences in colour are apparent in light which has travelled in directions 

 parallel to the crystallographic axes of the stone, that is to say, in three directions 

 perpendicular to each other, one of these being parallel to the edge of the prism shown in 

 Fig. 752. The light transmitted in one direction is a fine dark blue, in another, perpendicular 

 to this, a pale blue, while that travelling in the third direction is pale yellowish-grey or 

 almost colourless. The light which travels in intermediate directions is intermediate in 

 shade. When cordierite is examined with the dichroscope the images seen have very nearly 

 the colours just mentioned, and this feature is of importance in distinguishing cordierite 

 from other blue stones. 



In cutting cordierite the dichroism of the stone must be taken into account. According 

 to the orientation of the cut stone with respect to the crystal, there is produced a darker- 

 coloured lynx-sapphire or a lighter-coloured water-sanphire. The step-cut and the table-cut 

 are the forms most frequently adopted for cordierite ; but on account of the depth of colour 

 of this mineral the cut stone must not be too thick. The large front facet or table should 

 be perpendicular to the direction along which light of the darkest blue colour travels, so 

 that the stone will appear of the finest colour possible. As the plane of the table becomes 

 more nearly parallel to this direction, the stone becomes of a more and more dingy pale blue 

 or yellowish-grey colour. Cordierite is also cut en r.ahochon, and when this is the case the 

 same care in the orientation of the cut stone with respect to the crystal is necessary. 

 Cordiei'ites cut en cdbochon sometimes show a star of opalescent light similar to that 

 exhibited by star-sapphires. The stone may also be cut in such a manner as to exhibit its 

 dichroism to the fullest possible extent. This object is attained by cutting a cube, the 

 faces of which are perpendicular to the three axes of the crystal. The cube mounted by 

 one corner on a pivot, so as to show the three differently coloured faces, forms an interesting 

 and remarkable object. 



Cordierite is readily distinguished from sapphire, for the differences between the two 

 stones are many and well marked. The dichroism of sapphire is much less pronounced than 

 that of cordierite, while the hardness and specific gravity are much greater. Thus while 

 sapphire sinks in the four heavy liquids, cordierite, as arule, floats in all, though it may, in some 

 cases, sink in liquid No. 1. The same features distinguish it from other blue stones, such as 

 blue diamond, blue tourmaline, and kyanite, to be considered later, all of which are 

 appreciably denser. 



The occurrence of cordierite in nature i.s confined almost exclusively to granite and 

 gneiss ; it is found in some volcanic rocks, but only in small amount. Crystals regularly 

 developed on all sides, but too dark in colour for gems, occur embedded in granite at 



