OLIVINE 405 



sometimes present, as, for example, in Fig. 11 of Plate XIV., but they are very similar to 

 those jast described. Crystals of olivine are found not attached to the walls of drusy 

 cavities but embedded in the mother-rock ; they are therefore usually developed on all 

 sides. 



Olivine has no very distinct cleavage ; the best is in the direction of the large niacro- 

 pinacoid, and there is a very poor one parallel to the basal plane. The fracture is always 

 conchoidal. The hardness is slightly less than that of quartz, and is usually given as 

 H = 6f ; olivine is thus scratched by quartz, but easily scratches felspar 

 and still more easily ordinary window-glass. Though the hardness of 

 chi-ysolite compared with other precious stones is low, yet it is sufficient 

 to admit of the stone receiving a good polish ; the process of polishing 

 IS, however, not easy. On account of its comparative softness and its 

 liability to become scratched and dull this stone is not very highly 

 esteemed and is worn but rarely, especially as a ring-stone. 



The specific gravity of pure, transparent olivine ranges from 3'329 

 to 3-375. The greater the amount of iron present the higher is the 

 specific gravity ; hence the darker the colour the heavier the stone. fkj. 71. Crystalline 

 Heavy, dark-coloured olivine, therefore, sinks in pure methylene iodide, form of olivine (ohry- 

 but floats in liquid No. 4 (sp. gr. = 3-6). Stones of a light shade of ^°'"^^- 

 colour and low specific gravity are often of about the same weight, bulk for bulk, as 

 methylene iodide ; they will therefore remain suspended at any point in the liquid. A 

 stone having exactly this specific gravity will slowly sink to the bottom when the liquid 

 is warmed, even when the vessel containing it is held in the hand, and will rise to the 

 surface again when the liquid cools. Chrysolite may be readily distinguished from many 

 other green stones by its specific gravity. 



A certain amount of olivine is perfectly pure, clear, and transparent, and absolutely 

 free from faults of any kind. Material of this description is termed " noble chrysolite,'" and 

 this alone is cut for gems. The more abundant and more widely distributed " common 

 olivine " is cloudy and translucent to opaque, and is unsuitable for this purpose. The lustre 

 of olivine is of the ordinary vitreous kind, always, however, inclining to the greasy type ; its 

 brilliancy can be increased by polishing, as mentioned above. Being a rhombic mineral, 

 olivine is doubly refracting ; the index of refraction is not very high, but the strength of its 

 double refraction is very great, the double refraction of this stone being stronger than that 

 of any other precious stone except zircon. The least index of refraction for yellow light is 

 1"661 and the greatest 1'697 ; the difference between these two numbers, 0'036, is a measure 

 of the double refraction; the mean index of refraction is 1-678. The refractive indices 

 for light of other colours have not been determined ; they would not vary much, however, 

 from the values given above, since the dispersion is small and no brilliant play of prismatic 

 colours is shown by cut stones. 



We have already seen that the shade of colour shown by olivine depends upon the 

 amount of ferrous oxide present. The colour inclines now to yellow, now to brown, and 

 sometimes to green, but there is no marked difference between these shades, and they are 

 never very deep or intense. The usual colouring of chrysolite is shown in Figs. 11 and 12 

 of Plate XIV., the one representing a crystal and the other a cut stone. Common 

 olivine is not infrequently of a pronounced yellow colour. Specially named colour- 

 varieties of this stone are sometimes, but not universally, recognised. Thus chrysolite 

 proper is pale yellowish-green, peridot is deep olive-green, and olivine yellowish or light 

 olive-green. 



