Composition and Use of Garnets

GARNETS:

With materials which, though different in their nature, are cut and placed in the same manner, it is possible to construct several edifices resembling each other in form and disposition.

That which Art accomplishes more or less completely, Nature realizes perfectly.

Out of substances essentially differing in their nature she constructs well-defined and crystallized compounds, which appear to all intents and purposes identical, and yet are not so.

This phenomenon is called in science isomorphism, from the Greek isos, equal; and morphe, form. It was discovered by the celebrated German chemist Mitscherlich, and the discovery is considered one of the greatest scientific achievements of our century.

The group of minerals designated by the name of garnets furnishes one of the most remarkable applications of the grand theory of isomorphism.

Under this name are comprehended minerals differing much in colour, in specific gravity, in chemical composition, &c; but whose fundamental form never changes, and which even presents a very small number of secondary modifications. Garnets are always crystallized, and they appertain to the tesseral system. Two secondary forms only are nearly always produced, the rhombic dodecahedron (Fig. 80), and the trapezohedron (Fig. 81).

In a scientific classification Rose, and the greater number of mineralogists with him, admit eight species of garnets, but two only furnish products for jewelry; these are:--

The Grossularia.

The Almandine.

Grossularia.--This species is a double silicate of lime and alumina. As its constituents are colourless, either alone or combined, we should meet with limpid garnets devoid of any tint. Such garnets, in fact, are found in certain localities of Norway, Mexico, and the Ural Mountains. But as iron--which Hauy has called "the great colourist of nature"--is so extremely abundant, it becomes introduced in proportions more or less considerable into the grossularias, and the result is that the limpid garnets become tinted with green, clear red, orange red, &c., according to the quantity of iron combined. The varieties of Ala in Piedmont, so remarkable for the vivacity of their lustre and the purity of their forms, are grossularia. There are also certain yellow varieties in Siberia which resemble in colour the spinel ruby.

The analysis of a colourless garnet of the Ural has given the following results:--

Silica,.......... 38.66

Alumina,......... 24.19

Lime,............ 37.15

100.00

Almandine.--This species is a double silicate of alumina and iron; it is the grossularia with its lime replaced by oxide of iron. Often, however, the lime is not completely replaced, and the iron is not the only principle substituted, but then it is accompanied by an equivalent proportion of magnesia or oxide of manganese.

The beautiful variety of yellow garnet called pyrope belongs to the almandine species. It differs only from the type by a little quantity of the oxide of chromium taking the place of an equivalent quantity of the other bases. This substitution is regarded as perfectly regular by the mineralogist, but it produces a very agreeable colour, and gives commercially an altogether peculiar value to the pyrope.

It is to the almandine species also that the garnets so well known in commerce as Bohemian garnets belong. They are furnished by Bohemia, Saxony, and other parts of Germany.

The most desirable garnet is the oriental or Syrian garnet. Its composition varies, but its lustre and beauty place it above all others. Its name is not derived from Syria, as is often supposed, but from the Syrian, a river in the country called Pegu in Asia. It was from that country, indeed, that the first specimens were brought; but this commercial species is found equally in the Isle of Ceylon and in Brazil.

EXCEPTIONAL GARNETS, AND ARTISTIC APPLICATIONS OF THIS STONE.

At the sale of M. Dree's cabinet, a Syrian garnet of an octagonal form, 7/10 inch by 6/10, was sold for $650. Another, fire-red, 1 inch by 6/10 inch, attained the price of $186.