FLOOR.] 
MINERALS. 
193 
crystals assume rich hues, and when transparent become gems Div. IV. 
conspicuous for their extensive colour-suite, that rank next in 
value, as in lustre and hardness, to the diamond. These are the 
colourless Lux Sapphire, the (azure) Sapphire, the Ruby, the " Oriental- 
Topaz," " Oriental- Amethyst," "Oriental-Emerald," &c. ; gems not to 
be confounded with those from which they borrow their names, while 
distinguished from these by their title " Oriental," in allusion to the 
Eastern lands, India, Ceylon, Siam, Pegu, &c, which from the earliest 
times have produced the gem forms of this mineral in their greatest 
perfection. In the " Star stones " a six-rayed star is seen, of which 
the position is symmetrical in respect to the morphological axis of the 
crystal; and through the less pure varieties of Corundum, we descend 
to the opaque and granular, massive, but still, from their hardness, 
valuable states of this Mineral, of which Emery is an impure form. 
Identical in chemical and crystallographic type with Corundum, though 
very different in aspect both in its crystalline and massive varieties, Oases 17 
is the valuable iron ore, Haematite, the ferric oxide. A tarnish on and 18 - 
some of its crystals, especially on those from Elba, produces an irides- 
cent effect of great beauty. With Haematite is placed Ilmenite, or 
Titanic-iron, one of the ambiguous species of this class. Intimately 
blended with the former mineral in all proportions and crystallising 
in its forms, it yet presents the formula of titanate of Iron, a formula, 
however, which, as containing two equivalents of metal united to three 
of oxygen, is in fact homotypic with the sesquioxides. 
The hydrates of this class include the important iron ore Limonite 
(Brown-hasmatite), and Goethite, w T hich is monohydrated ferric oxide. Cases 18 
In juxtaposition with the fine Cornish specimens of this mineral/ h\) and 19. 
from the Restormel mine, are Manganite and Diaspore, respectively 
the monohydrated manganese and aluminium oxides, isomorphous with 
Goethite. 
The class of dioxides is illustrated by a series of crystals and 
other forms, — especially rich in the Cornish varieties — of Cassiteritecase 29. 
or Tin-stone, the ore of tin: and in the same Case is placed the 
Zircon, consisting of the associated zirconium and silicon dioxides. 
Its crystals, like those of Cassiterite, with which it is nearly isomor- 
phous, are pyramidal. Its pellucid varieties are gems. The dull 
green is the Jargoon, while peculiar (" hyacinthine") red tints 
characterize the gem known as the Hyacinth or Jacynth, of which fine 
cut specimens are in Case 20 (iv.). The yellow and blue tints are 
rare, but the more pellucid and colourless zircon, from its exception- 
ally high refractive power, approaches even the diamond in brilliancv. 
In the same continuous series is Rutile, the titanium dioxide 
isomorphous with Zircon, and approximately so with Cassiterite. Case 21. 
Anatase is the same substance, also in pyramidal forms, but with dif- 
ferent parameters ; while in yet a third series of forms this trimorphous 
titanic dioxide is to be seen a^ the orthorhombic mineral Brookite, 
of which the specimens from the Snowdon district are remarkable. 
In this Case is also exhibited the manganese dioxide (the "per- 
oxide"). Pyrolusite, the mineral employed for the production of 
o 
