NON-METALLIC MINERALS. 
115 
same crystal is well seen in the parti- coloured tourmalines from 
Elba, and from Maine, U.S.A. 
The only other mineral of popular interest in this Case is the 
Topaz. This is a silicate of alumina associated with a silico- 
fluoride of aluminium. The topaz is found in the form of rolled 
pebbles, and in granitic rocks, commonly associated with quartz,, 
and not unfrequentiy with tin ore. The yellow and the white 
topazes, chiefly from Brazil, form valuable gems ; and in many 
cases a pinkish tint is developed by exposure to heat. The 
topaz of the ancients appears to have been the stone which we 
now call Chrysolite ; this is a silicate of magnesium of a dull 
green colour, known also as Peridot, the name Olivine being 
applied to the less transparent varieties, commonly found in 
meteorites, and in basalt and similar rocks. Examples of these 
varieties are exhibited in the next Case, S. 
Case S. — Zircon, Garnet, &c. 
Many of the silicates exhibited in this Case are, like those in 
the last section, employed for purposes of ornament. Zircon is 
a silicate of the rare metal zirconium. The transparent coloured, 
zircons are used as gems ; the rich red varieties being distin- 
guished as Hyacinth or Jacinth, and the less brightly coloured 
as Jargoon. The hyacinth occurs in the form of rolled crystals, 
chiefly in Ceylon and central France ; whilst the coarse dull- 
coloured Zirconite forms a constituent of the zircon-syenite of 
Norway, and i some other rocks. Microscopic zircons are not 
uncommon in such, rocks as granite. 
The group of Garnets embraces a considerable number oF 
minerals which are essentially double silicates, the varieties 
depending on the character of the bases. The deep- coloured. 
Almandine or precious garnet, a silicate of aluminium and iron,, 
is frequently cut en cabochon, when it is known as Carbuncle ; 
and the Essonite or cinnamon stone is also used as a gem, being 
often mistaken for hyacinth. Essonite is a silicate of aluminium 
and calcium, thus having a composition similar to that of the 
Siberian Grossularia or gooseberry garnet. Other Russian 
garnets are the demantoid, a stone of great brilliancy, when 
cut, and the Uivarowite, a mineral having a bright emerald- 
green colour, and containing the silicates of calcium, aluminium, 
and chromium. The species Idocrase or vesuvian has a com- 
position identical with that of certain garnets, but crystallises, 
in forms totally distinct. 
Case T. — Felspars. 
The minerals in this Case, although less attractive than those 
in the preceding sections, are nevertheless of the highest interest 
as rock -constituents. The Case is devoted to the family of 
Felspars, which includes a number of species, of which the best 
