MINERALOGY. 



85 



Gpitrit- The next class in the section of oxides is composed of minerals of a 

 ss. chemical type, similar to that of the magnetic oxide of Iron (the 

 ferrous-ferric oxide), which may in fact be viewed as a combination of 

 ferrous oxide with ferric oxide, and thus, while possessing as an epi- 

 tritoxide the formula M 3 0 4 "and a place in the section of the oxides, 

 has claims to be recognised as a salt. 



The group of cubic-formed minerals to which Magnetite more espe- 

 cially belongs, the " Spinel Group," includes Franldinite 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 Kuby " 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 combina- 

 tion of glucina and alumina (aluminate of Glucinum), homotypic 

 with epitritoxides. It is prismatic in crystallisation, and as a gem, 

 known by the name of " oriental chrysolite," it presents itself as a 

 beautiful greenish-yellow stone, almost equal in lustre and in hard- 

 ness to the Sapphire. It also has the name Cymophane, from a cloudy 

 appearance that presents itself in two of the planes of the crystal, and 

 is retained even when 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 the dark green variety, termed Alexandrite, very fine specimens 

 are seen in Case 9e. It is amethyst-coloured by artificial light. 



sesqui- The next class among the oxides is that of the sesquioxides. 



I The pure oxide of Aluminium is seen in colourless crystals of 

 Corundum, consisting for the most part of hexagonal pyramids 

 and prisms. With minute traces of colouring ingredients, these 

 crystals assume rich hues, and when transparent become gems 

 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-Anetlryst," " 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, 



