416 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



yellowish-brown colour are sometimes met with. In reflected light the green pebbles usually 

 appear of a dark shade of colour, but some are pale grass-green tinged with yellow. They 

 are strongly dichroic, and this feature is distinctly observable even with the naked eye. 

 When viewed in the direction of the two horizontal axes, indicated in the pebbles by the 

 cleavage cracks, two shades of green are seen, one being rather more inclined to yellow than 

 the other; when, on the other hand, the pebble is viewed through in a direction parallel to 

 the prism-edge it appears of a pretty brownish-red colour. The pairs of images seen in the 

 dichroscope vary in colour between these principal shades. The two colours 

 characteristic of andalusite are therefore the same as those shown by another 

 dichroic precious stone, namely, alexandrite. Although the green of the alexandrite 

 is more of an emerald-green and the brownish-red is deeper and more intense, yet 

 the similarity between the two stones is close enough to admit of andalusite being 

 passed off' as the more costly alexandrite. The latter may readily be distinguished 

 from andalusite by its greater hardness and higher specific gravity, the density of 

 alexandrite being 3-64 while that of andalusite varies between 3"17 and 3'19. 



In cutting andalusite, as in the case of other dichroic precious stones, the 

 orientation of the cut stone relative to the crystal must be considered. It may be 

 cut in such a way as to show one of the principal colours, or, on the other hand, so 

 that both colours may be displayed. Cut stones are but little used and their price 

 is low. The lustre of the mineral is of the vitreous type, and is but little increased 

 by polishing. The hardness, H = 7 J, is slightly greater than that of quartz. Like 

 kyanite, andalusite is infusible before the blowpipe, and is unattacked by acids. 



The peculiar variety of andalusite distinguished as chiastolite (cross-stone) 

 should not be forgotten. It has all the essential characters of andalusite, but occurs 

 in elongated, irregular prisms up to an inch in thickness, which are always embedded 

 in dark clay-slates. The peculiarity of the stone depends on the fact that the prisms 

 are not only surrounded but also penetrated to a greater or lesser depth by the 

 clay-slate. Thus, a rod of the clay-slate varying in thickness in different parts runs 

 centrally throughout the whole length of the crystal. This is connected with the four 

 portions which penetrate the crystal at its comers, so that a cross-section of the 

 prism shows a dark cross on a white background (Fig. 77). The figure shows a 

 number of cross-sections of a single prism arranged serially. From these it is 

 Tig. 77. apparent that the enclosed clay-slate increases in amount from below upwards, 

 the white andalusite substance predominating in the lower, and the dark clay-slate 

 in the upper portion of the prism. 



It is on the appearance presented by cross-sections of the prism that the value, such 

 as it is, of chiastolite is due, for, in certain places, especially in the Pyrenees, such sections 

 are worn as amulets and charms. Suitable crystals of sufficient size are to be met with at 

 several localities in this neighbourhood ; also at Salles de Rohan, near Brieux, in Brittany, 

 and at many other places. Recently considerable numbers of fine, large crystals of 

 chiastolite have been found in clay-slate at Mount Howden, ten miles north of Bimbowrie, 

 in South Australia ; these are of large size, reaching a length of 6 inches and a diameter 

 of 2 inches ; the cross-sections take a good polish and show cross-figures of various forms. 



Chiastolite is a contact-mineral which occurs in abundance in clay-slates near their 

 contact with granite. It is found, for example, at Hof, in the Fichtelgebirge, and in the 

 slates round the Skiddaw granite in Cumberland, but it usually takes the form of thin 

 needle-like prisms, which are not thick enough for the purpose mentioned above. Crystals 

 as large as those found in the Pyrenees are rare. 



