SPECIAL OPllCAL APPEARANCES AND COLOUR EFFECTS 63 



peculiar and unusual conditions present in each case. The exact nature of these conditions, 

 and the manner in which they cause the abnormal appearances, is not completely known in 

 every case ; as full an explanation as is possible will be given in the description of each 

 particular case. 



The play of prismatic colours exhibited by the diamond has already been dealt 

 with in detail ; we have here the simplest case of the refraction and dispersion of light. 



The prismatic colours produced by cracks in the interior of a transparent stone, and 

 best shown in colourless examples, gives rise to the appearance usually known as 

 iridescence. The irregular fissures, or more frequently the plane cleavage cracks, inside 

 a crystal represent narrow crevices, which may be vacuous or filled with air; these films 

 give rise to the brilliant prismatic colours known as Newton's rings, or as the colours of thin 

 films or plates. These colours, which are shown to perfection by soap-bubbles, are inde- 

 pendent of the colour of the substance itself, or of any colouring-matter contained in it, but 

 are due to purely physical causes connected with the passage of white light through the film. 

 The phenomenon which thus gives rise to the appearance of prismatic colours is known to 

 physicists as the interference of light. 



Some iridescent stones, such as rock-crystal, are occasionally cut so as to bring the 

 crack, to which the display of prismatic colours is due, near the surface, and thus render 

 them the more striking. Especially beautiful prismatic colours are shown by some kinds of 

 colourless opal, namely, the so-called noble or precious opal (Plate XVI., Figs. 6-9). These 

 colours are not shown over the whole surface of the stone, but in small discontinuous patches, 

 closely aggregated. This appearance in opal is certainly a kind of iridescence, but as to its 

 exact cause there is still a difference of opinion. 



The translucent or semi-transparent variety of potash-felspar or orthoclase, known as 

 adularia, sometimes shows a bluish or milky reflection of light, not from the whole surface 

 but only from certain crystal lographic planes. This opalescent appearance is specially 

 prominent when the stone is cut and polished with a rounded convex surface, over which, 

 when the gem is moved, a streak or wave of such reflected light passes. This appearance, 

 being specially pronounced in adularia, is sometimes known as adularescence. It has 

 been compared to the soft light of the moon, and specimens showing it to perfection are 

 called moon-stones (Plate XVI., Figs. 4 and 5), and are often used for ornamental purposes. 

 The pearly opalescence of adularia is due to reflection of light from internal platy fractures 

 or planes of separation, and from microscopically small crystal plates embedded in the 

 adularia along these planes. 



A similar appearance is shown by some specimens of chrysoberyl, which are also valued 

 as precious stones under the name of cymophane or cat's-eye (Plate XII., Fig. 11), since the 

 sheen of this green, yellowish-green, or brown stone recalls the appearance of the eye of the 

 cat. We shall see later on that there is a variety of quartz having this same appearance ; 

 the chrysoberyl variety is therefore distinguished as true or " oriental cat's-eye." 



The brilliant colours shown on certain faces of labradorite, a felspar from Labrador 

 (Plate XVI., Fig. 2), as well as by a potash-felspar from Fredriksvarn in southern Norway 

 (Plate XVI., Fig. 3), is also, like adularescence, due to the presence of numerous minute 

 crystal plates enclosed in the felspar and arranged parallel to these planes. The appearance 

 resulting from the peculiar structure of these minerals is known as change of colours or 

 labradorescence. In most positions these minerals are dull grey and unattractive 

 looking, but certain faces in reflected light, and at a certain inclination to the light, show 

 the most brilliant shades of green, blue, violet, red, yellow, &c. The small plates, which 

 o-ive rise to the reflection of coloured light, may be seen under the microscope embedded in 



