OXIDES. 
85 
and black colour, lias 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, Flint (15b), the fracture is con- 
choidal, 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. 
The Heliotrope, or Bloodstone (16a), is a green stone with red 
blood-like spots. 
'Next follow the Plasma and Chrysoprase, green stones : and 
the Sard, generally a brownish red ; as also the Sardonyx, its 
banded variety : all of them 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 are 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. Carnelian (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 wliich is a 
fine suite from Queensland, presented by Professor Story- 
Maskelyne. 
