52 
NORTH GALLERY. 
[UPPER 
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,' 4 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 r 
valuable states of this Mineral, of which Emery is an impure form. 
Identical in chemical and crystallographic type with Corundum, though 
Cases 17 very different in aspect both in its crystalline and massive varieties, 
and 18. i s the valuable iron ore, Haematite, the ferric oxide. A tarnish on 
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 
Oases 18 (Brown-haematite), and Gcethite, which is monohydrated ferric oxide. 
(ii.)and 19. -"- n juxtaposition with the fine Cornish specimens of this mineral, 
from the Restormel mine, are Manganite and Diaspore, respectively 
the monohydrated manganese and aluminium oxides, isomorphous witli 
Gcethite. 
The class of dioxides is illustrated by a series of crystals and 
Ca«e 9 0 °ther forms, — especially rich in the Cornish varieties — of Cassiterite 
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 duli 
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 brilliancy. 
In the same continuous series is Rutile, the titanium dioxide 
9] isomorphous with Zircon, and approximately so with Cassiterite. 
(i.) 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 as the orthorhombic mineral Brookite,. 
of which the specimens from the Suovvdon district are remark« 
able. 
In this Case is also exhibited the manganese dioxide (the "per 
oxide"), Pyrolusite, the mineral employed for the production of 
