DICHROISM -^9 



sometimes be restored by keeping the stone in darkness, by burying it in moist earth, or by 

 treating it with certain chemicals, all of which devices are made use of by unscrupulous 

 dealers. As a contrast to the behaviour of such stones on exposure to light, it may be 

 mentioned here that amber, instead of being bleached by exposure, is darkened, gradually 

 becoming of a dark, reddish-brown colour. 



The artificial colouring or recolouring of precious stones, which was known and 

 practised to some extent among the ancients, is of some importance. At the present day 

 agate and similar stones are most frequently subjected to this treatment, the exact methods 

 of which will be dealt with under the special description of these stones. The capacity for 

 absorbing the liquid which imparts its colour to the stone, often even to the central portions, 

 depends on the porous nature of its substance. 



Streak. — In speaking above of idiochromatic and allochromatic minerals, we have seen 

 that in the former the fine powder of the mineral is also coloured, the colour being charac- 

 teristic of the mineral. For the purpose of quickly and easily obtaining a mineral in the 

 state of fine powder, it is rubbed on a plate of rough, unglazed, " biscuit " porcelain. The 

 line of powder left upon the plate by the mineral is known as its streak, the colour of which 

 can be easily observed on the white background. The streak is often characteristic of a 

 mineral, and thus the observation of the streak is a step towards the determination of the 

 mineral. The character is not of much practical value in the determination of precious 

 stones, since on account of their hardness they are much more likely to scratch the porcelain 

 than to leave a streak upon it ; moreover, the streak of most precious stones, as in other 

 allochromatic minerals, is white, and therefore not a distinguishing feature. 



6. DiCHllOISM. 



An important optical property of many precious stones is that known as dichroism or 

 pleochroism. A stone possessing this property, when observed in different directions will 

 show different colours or shades of colour which may resemble each other more or less closely, 

 or may, on the other hand, differ considerably. A mineral sometimes used as a cut stone, 

 and known as " water-sapphire," exhibits this phenomenon to such a marked degree that it 

 has received the name dichroite, although at the present time it is usually known to miner- 

 alogists as cordierite. A crystal of this mineral, when viewed in three particular directions, 

 perpendicular to each other, appears of three distinct colours, namely, a fine dark blue, light 

 blue, and greyish-yellow. In intermediate directions are seen intermediate tints, which 

 approach one or other of the three principal colours according to the direction of the line of 

 view. The three particular directions along which these maximum differences in colour are 

 observable are definitely related to the crystalline form of the mineral ; they are in fact the 

 three crystallographic axes of the rhombic crystal. 



The difference in colour shown by cordierite when viewed in different directions is very 

 great, but it is perhaps even greater in some kinds of tourmaline. In this mineral the 

 colour, as seen in different directions through a crystal, varies from yellowish-brown to 

 asparagus-green, or in other crystals (of the same mineral) between dark violet-brown and 

 greenish-blue, or again in others between purple-red and blue, &c. Some dichroic precious 

 stones show only very small differences in colour when viewed in different directions ; the 

 yellowish-green chrysolite is an example. Indeed the majority of pale coloured stones are 

 only feebly dichroic, stronger dichroism being exhibited by minerals having a deeper tone 



of colour. 



Finally there are other minerals, such as garnet and spinel, which show no differences of 



