Garnet is a prominent constituent of many kinds of rocks, but the material most suitable for gem purposes occurs chiefly in crystalline schists or metamorphic limestones. Pyrope and demantoid are furnished by peridotites and the serpentines resulting from them; almandine and spessartite come mostly from granites.
The name of the species is derived from the Latin granatus, seed-like, and is suggested by the appearance of the spherical crystals when embedded in their pudding-like matrix.
The varieties most adapted to jewellery are the fiery-red pyrope and the crimson and columbine-red almandine; the closer they approach the ruddy hue of ruby the better they are appreciated. Hessonite was at one time in some demand, but it inclines too much to the yellowish shade of red and possesses too little perfection of transparency to accord with the taste of the present day. Demantoid provides beautiful, pale and dark emerald-green stones, of brilliant lustre and high dispersion, which are admirably adapted for use in pendants or necklaces; on account of their comparative softness it would be unwise to risk them in rings. In many stones the colour takes a yellowish shade, which is less in demand. Uvarovite also occurs in attractive emerald-green stones, but unfortunately none as yet have been found large enough for cutting. A few truly magnificent spessartites are known—one, a splendid example, weighing 6¾ carats, being in the possession of Sir Arthur Church; but the species is far too seldom transparent to come into general use. The price varies per carat from 2s. for common garnet to 10s. for stones most akin to ruby in colour, and exceptional demantoids may realize even as much as £10 a carat. The old style of cutting was almost invariably rounded or en cabochon, but at the present day the brilliant-cut front and the step-cut back is most commonly adopted.
The several varieties will now be considered in detail.
(a) Hessonite
(Grossular, Cinnamon-Stone, Hyacinth, Jacinth)
This variety, strictly a calcium-aluminium garnet corresponding to the formula Ca3Al2(SiO4)3, but generally containing some ferric oxide and therefore tending towards andradite, is called by several different names. In science it is usually termed grossular, a word derived from grossularia, the botanical name for gooseberry, in allusion to the colour and appearance of many crystals, or hessonite, and less correctly essonite, words derived from the Greek ἥσσων in reference to the inferior hardness of these stones as compared with zircon of similar colour; in jewellery it is better known as cinnamon-stone, if a golden-yellow in colour, or hyacinth or jacinth. The last word, which is indiscriminately used for hessonite and yellow zircon, but should more properly be applied to the latter, is derived from an old Indian word (cf. [p. 229]); jewellers, however, retain it for the garnet.
Only the yellow and orange shades of hessonite ([Plate XXIX], Fig. 5) are used for jewellery. Neither the brownish-green kind, to which the term grossular may properly be applied, nor the rose-red is transparent enough to serve as a gem-stone. Hessonite may mostly be recognized, even when cut, by the curiously granular nature of its structure, just as if it were composed of tiny grains imperfectly fused together; this appearance, which is very characteristic, may readily be perceived if the interior of the stone be viewed through a lens of moderate power.
The specific gravity varies from 3·55 to 3·66, and the refractive index from 1·742 to 1·748. The hardness is on the whole slightly below that of quartz. When heated before a blowpipe it easily fuses to a greenish glass.
The most suitable material is found in some profusion in the gem-gravels of Ceylon, in which it is mixed up with zircon of an almost identical appearance; both are called hyacinth. Hessonites from other localities, although attractive as museum specimens, are not large and clear enough for cutting purposes. Switzerland at one time supplied good stones, but the supply has long been exhausted.