OXIDES. 89 



and black colour, has been used from remote ages to test the 

 purity of precious metals. 



Hornstone (15a) is a variety of silica without evident 

 crystallisation, and generally presents a more or less splintery 

 fracture ; but in one kind, Mint (15b), the fracture is shell- 

 like (conchoidal), but sometimes conical, as is well shown by 

 specimens in the case. 



Chalcedony (15b) has a lustre nearly that of wax, and is 

 either transparent or translucent. The specimens from 

 Uruguay (15d) are of especial interest as containing imprisoned 

 water. 



Heliotrope, or Bloodstone (16a), is a gi-een stone with red 

 blood-like spots. 



Next follow the Plasma and Chrysoprase, which are green 

 stones : and the Sard, generally brownish red ; as also the 

 Sardonyx, its banded variety. All were much prized by the 

 ancients because though hard and tough enough to resist 

 ordinary wear and tear, they are more suited to the display of 

 the engraver's skill than the harder and more precious stones. 



Then come the Agates (16b), chiefly formed of thin layers of 

 porous chalcedony of different colours, though the material of 

 many of the white layers is a compact semi-opal. Most are 

 now brought from Uruguay, in South America, and are cut and 

 polished at Oberstein, in Germany, where in former times 

 agates were collected in quantity from the mountains of the 

 district. Sometimes the layers are parallel, and the stone is 

 then an Onyx, useful as a material for cameos : or the bands of 

 a section are arranged in parallel sets of zigzag lines, and the 

 stone is then called a Fortification-agate ; but in the ordinary 

 agate the layers are variously curved. 



Moss-agates, or Mocha-stones (16e), are varieties of chalce- 

 dony enclosing moss-like forms of oxides of manganese and 

 iron and green earthy chlorite. CameUan (16e) is a beautiful 

 stone much valued by the engraver : its fracture has a peculiar 

 waxy lustre, and~is distinct from that of the Sard, which is 

 dull and horn-like. Next follows Opal, including the Precious 

 or Noble Opal (16f), among the specimens of which is a 

 fine suite from Queensland presented by Professor Story- 

 Maskelyne. 



