580 



DETERMINATION OF PRECIOUS STONES 



Iron-pyrites differs from all the other minerals of this group in the possession of a high 

 specific gravity and a metallic lustre. 



Carnelian varies in colour from yellow to yellowish-brown and reddish-brown ; when of 

 a pronounced chestnut-brown colour it is known as sard. It frequently exhibits alternating 

 bands of different colours. Opal, though similarly coloured, is never banded, and is less 

 hard and less dense than is carnelian. Natrolite is of an isabel-yellow colour inclining to 

 brown ; it has but little lustre and is always fibrous in structure. Amber feels warm to the 

 touch, acquires a strong charge of electricity when rubbed, and can be cut with a knife ; it 

 is thus easily distinguished from glass. 



Glass is always heavier than natrolite or fire-opal, and is readily scratched by 

 carnelian. 



6. ROSE-RED, RED, AND LILAC STONES, 



Rhodonite is rose-red, but of a darker shade than rose-quartz, which is less dense, 

 harder, and more lustrous than rhodonite. 



Lepidolite is of a lilac shade and may be scratched even with the finger-nail, and still 

 more readily with a knife. 



Jasper is perfectly opaque and ranges in colour from dark red to brownish-red. Car- 

 nelian is translucent and usually of a darker or lighter shade of yellowish-red. Fire-opal is 

 sometimes very similar in colour to carnelian, but is lighter and less hard. 



Glass is softer than any member of this group with the exception of lepidolite. 



7. STONES EXHIBITING MORE THAN ONE COLOUR. 



The curved concentric layers of which malachite is built up appear, in section, as bands 

 coloured alternately light and dark green, the latter, indeed, so dark as to be sometimes 

 almost black. It is distinguished from the other minerals of this group by its high specific 



