46 
NORTH GALLERY. 
[UPPER 
ferrous-ferric oxide), which may in fact be viewed as a combination of 
ferrous oxide with ferric oxide, and thus, wbile possessing as an epi- 
tritoxide the formula and a place in the section of the oxides, has 
claims to be recognized as a salt. 
Case 16. The group of cubic-formed minerals to which Magnetite more espe- 
O'i-) cially belongs, the " Spinel Group," includes Franklinite and Chro- 
mite (Chromic-iron), which latter mineral is the source of the chrome 
yellow and of some other colouring matters employed in the arts. The 
Spinels, properly so called, also belong to it. These are aluminates 
of Magnesium, also of Zinc, Iron, and Manganese : ferric oxide, 
too, occasionally plays the part of alumina. The deep-red " Spinel 
Euby " and the pale rose-tinted " Balas Euby" are beautiful gems 
cut from specimens of this Mineral, of which a good assortment of 
crystals is exhibited. Pleonast, Gahnite, Dysluite, are opaque varieties 
of Spinel. To this class also may be referred the Chrysoberyl, a com- 
bination of glucina and alumina (aluminate of Glucinum), homotypic 
with epitritoxides. It is prismatic in crystallization, and as a gem, 
Case 16. known by the name of " oriental chrysolite," it presents itself as a 
(fr.) beautiful greenish yellow stone, almost equal in lustre and in hardness 
to the Sapphire. It also has the name Cymophane, from a cloudy ap- 
pearance that presents itself in two of the planes of the crystal, and is 
retained even w r hen the transparent stone is cut and polished. Cut en 
cabochon, the less transparent specimens furnish one of the kinds of 
stone to which the jewellers give the name of Cat's-eye. The dark 
green variety from the emerald mines of the Ural exhibits trichroism, 
absorbing the different colours in different amounts according to the 
crystallographic direction the light pursues on entering the crystal. 
Of this variety, termed Alexandrite, very fine specimens are seen in 
this Case. 
Case 17. The next class among the oxides is that of the epideutoxides (the 
so-called " sesquioxides"). The pure oxide of Aluminium is seen in 
colourless crystals of Corundum, consisting for the most part of hexa- 
gonal pyramids and prisms. With minute traces of colouring ingre- 
dients, these crystals assume rich hues, and become gems conspicuous 
for their extensive colour-suite, and ranking 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-Ame- 
thyst,' 'Oriental-Emerald,' &c. ; gems not to be confounded with those 
from which they borrow their names, while distinguished from these 
b7 their title " Oriental," in allusion to the Eastern lands, of 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 to play, its position being sym- 
metrical in respect to the axis of the crystal ; and through the less pure 
varieties of Corundum, we descend to the opaque and granular, mas- 
sive, but still, from their hardness, valuable states of this Mineral, of 
which Emery is an impure form. Identical in chemical and crystallo- 
< a 17 graphic type with Corundum, though very different in aspect both in 
and 18. its crystalline and massive varieties, is the valuable iron ore, Haematite, 
