COLOUR 57 



organic or inorganic, and differ much in character. They may exist in considerable amount, 

 but more frequently they are present in such small quantities that very exact chemical 

 analysis is necessary for their detection. In the latter case, the colouring-power of the 

 pigment must be comparable to that of carmine and some other pigments, an extremely 

 small quantity of which is capable of giving a decided colour to an enormous quantity of a 

 colourless substance. 



The precise nature of the colouring-matter of many precious stones it has been 

 impossible as yet to determine. Large quantities of the precious stones would be needed to 

 yield an amount of colouring-matter sufficient for a reliable analysis, and here lies the chief 

 obstacle to the investigation. In spite of this difficulty it has been possible in some cases to 

 determine definitely to what substance the colouration is due. Thus, for example, the 

 emerald owes its green colour to the presence, in small quantity, of a compound of the 

 metal chromium, while the apple-green colour of chrysoprase is due to a compound of the 

 metal nickel. Other stones are coloured by compounds of iron or copper ; while the brown 

 colour of smoky-quartz is due to an organic substance, which can be distilled off as a dark 

 brown oil, possessing an empyreumatic odour. 



The colouring-matter of precious stones is frequently distributed so intimately and 

 uniformly through their substance that it is impossible, with the strongest magnification, 

 to distinguish single particles of the pigment. The relation between the substance of the 

 precious stone and the pigment seems analogous to that which exists between a solvent and 

 a substance dissolved in it. In such cases, it is inferred that the colouring-matter is not an 

 essential constituent of the substance of the stone from the fact that specimens of other 

 colours, or devoid of colour, are known. This intimate association of the pigment with the 

 ground substance of the stone exists, for example, in the green emerald, in the blue opaque 

 turquoise, the colour in the latter case being due to compounds of copper and iron ; as also 

 in diopside, the green colour of which is given by a compound of ferrous oxide. 



In most of these cases we are dealing with something; more intimate than a mere 

 mechanical mixture. The colouring-matter is isomorphous with the ground substance of the 

 precious stone, that is, it has the same type of chemical formula, and when this is the case, 

 the intermixing of the two substances involves not microscopically small particles of each 

 but the ultimate particles or molecules of each substance. Thus to take diopside as an 

 example, we must picture the molecules of the compound of ferrous oxide distributed 

 uniformly between the molecules of the ground substance of the stone and. imparting to it 

 its characteristic green colour. The same may perhaps be said of emerald which belongs to 

 the mineral species beryl, specimens of which sometimes occur colourless ; also of turquoise 

 and many other precious stones. 



In other stones, on the contrary, the colour is due to vast numbers of minute coloured 

 particles, with definite boundaries, mechanically intermixed with the colourless ground-mass 

 of the stone. These particles may be large enough to be just perceptible to the naked eye, 

 or so small as to require a lens or microscope for their detection ; they may have the form 

 of grains, scales, fibres, or needles. Small blue grains distributed in large numbers through 

 the colourless ground-mass of lapis-lazuli give to this precious stone its fine blue colour. 

 Green needles and fibres of the mineral actinolite give rise to the green colour of prase, a 

 variety of quartz, which of itself is colourless. Felspar is sometimes coloured red by minute 

 scales of iron-glance (haematite), and is then used as an ornamental stone under the name of 

 sun-stone : chalcedony, coloured by a similar red pigment, is the much used carnelian. 



, Stones coloured in this manner, by the mechanical intermixture of particles of pigment, 

 are more or less cloudy or even opaque ; those, on the contrary, in which a more intimate or 



