FLOOll.] 
MINERALS. 
51 
gems conspicuous for their extensive colour-suite, that rank next in Div. IV. 
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 G-^'^Q^ 17 
very different in aspect both in its crystalline and massive varieties, 
is 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 an epideutoxide (sesquioxide). 
The hydrates of this class include the important iron ore Limonite Cases 18 
(Brown-haematite), and Goethite, which is monohydrated ferric oxide, (ii.)aiid 1J>. 
In 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 with 
Goethite. 
The class of dioxides is illustrated by a series of crystals and Case 20. 
other forms, — especially rich in the Cornish varieties — of Cassiterite 
or Tin-stone (stannic acid), the ore of tin : and in the same Case is 
placed the Zircon, consisting of the associated zirconic and silicic 
dioxides (zirconic and silicic acids). Its crystals, like those of Cassi- 
terite, with which it is nearly isomorphous, 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 colour- 
less zircon, from its exceptionally high refractive power, approaches 
even the diamond in brilliancy. 
In the same continuous series is Rutile, the titanium dioxide Case 21 
(titanic acid), isomorphous with Zircon, iinol approximately so with (i ) 
Cassiterite. Anatase is the same substance, also in pyramidal forms, but 
with different parameters ; while in yet a third series of forms this tri- 
morphous titanic dioxide is to be seen as 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 
E 
