TOURMALINE 367 



of the isomorphous series of minerals, known collectively as tourmaline, are due to the 

 association of these molecules in different relative proportions. 



When a tourmaline contains, beside its constant constituents, small amounts of 

 manganese and lithium with little or no iron, it is usually colourless, rose-red, darker 

 red, or light green according, apparently, to the proportions in which iron and manganese 

 are present. The darker red shade probably appears when a relatively considerable amount 

 of manganese is present, while with a rather larger amount of ferrous oxide the colour 

 is dark green, as is so often the case in silicates. Some green tourmalines, however 

 contain chromic oxide, the substance to which the emerald owes its magnificent gi'een 

 colour. 



In brown tourmaline there is practically no iron, manganese, or lithium, but a much 

 larger amount of magnesium than is present in other varieties. The presence of a large 

 amount of ferrous oxide is probably the cause of a blue colour, but apparently no 

 transparent blue tourmaline has as yet been chemically analysed. The tourmaline richest 

 in iron is, as has been stated, black and opaque in mass, but in thin sections is transparent 

 and brown, green, or blue, in colour. The colours of tourmaline withstand the action of 

 heat very markedly, in many cases remaining essentially unaltered after exposure to a 

 red-heat. For example, dark -green stones after this treatment are still green, but of a 

 pale or greyish shade, a change which renders them unsuitable as gems. 



The dichroism of tourmaline is a very prominent feature ; it is apparent in quite pale- 

 coloured stones, but is more marked in the darker varieties. Tourmaline is, in fact, more 

 strongly dichroic than any other precious stone, with the exception perhaps of cordierite 

 (dichroite), but this is seldom cut as a gem-stone. In all cases the colour of the light which 

 travels through a crystal of tourmaline in a direction parallel to the prism edges — that is to 

 say, along the principal or optic axis — is darker than that which travels in a direction 

 perpendicular to this. Again, a moderately thick slice of a dark-coloured crystal cut 

 perpendicular to the optic axis may be quite opaque, while a plate of the same thickness, 

 but cut parallel to the optic axis, may be transparent. The colours visible in these two 

 directions are also usually different, as has been explained in the general account of 

 dichroism given in the first part of this book. Very frequently it is possible to observe 

 this difference in colour with the naked eye, unaided by the dichroscope. The maximum 

 difference in colour of the two images of the dic:hroscope aperture will be seen when the 

 crystal is viewed through in a direction perpendicular to the prism edges ; one image 

 will, as a rule, be darker than the other, and the particular colours shown will depend 

 on the colour of the crystal. Thus with brown crystals the two images will be dark 

 brown and pale brown to yellow ; with red crystals they are darker and lighter red, and so 

 forth. If the crystal is deeply coloured, one of the images will be almost or quite black 

 and the other a light shade of the colour shown by the crystal. 



In consequence of the strong dichroism of tourmaline, it is necessary that a crystal with 

 a considerable depth of colour should be cut in a certain direction, namely, with the plane 

 of the table or large central facet parallel to the prism edges. By this means the light 

 which reaches the eye of the observer from the stone has travelled through it mainly in 

 a direction perpendicular to the principal or optic axis of the crystal, thereby ensuring for 

 the stone the best possible appearance. If, on the other hand, the stone is cut so that the 

 table is perpendicular to the prism faces, that is to say, parallel to the basal plane, then the 

 light will ti-avel through the stone mainly in a direction approximating to that of the 

 principal axis of the crystal, and the gem will have a dull, cloudy, and unpleasing appearance. 

 Some of the Brazilian tourmalines, when cut in the manner first described, display a fine green 



