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ENGRAVED GEMS.
47

enamel—the sum of the effects due to lustref texture, form, size, etc., as well as the balance and distribution of colour.

In any treatment, however cursory, of the topic of this chapter, the artistic employment of precious stones, some reference ought to be made to the materials used by the gem-engraver. Nearly all the minerals employed for intaglios and cameos will be found mentioned in a subsequent chapter. Most of them are varieties of silica coloured by small quantities of iron-compounds. Such are the sards, cornelians, onyxes, chalcedonies, amethysts and jaspers in which the great majority of antique gems were wrought. An intaglio well engraved in one of the more transparent or translucent of these stones, say, on a rich golden of blood-red sard, shows effects of beautiful colour when viewed by transmitted light which will be sought for in vain in any faceted specimen. And then the cameos of later dates, wrought in onyx and sardonyx, present delightful contrasts of tone and hue in their different strata, utilised as these layers often were in the building up of a relief-picture. Of other minerals employed for engraving in classic times mention may be made of beryl, garnet and plasma; the harder and rarer stones were, however, little used until medieval and later days. It is well to remember that the jacinth, properly so-called, that is, the orange brown or brownish red zircon, has never yet been found with an engraving of classic date upon it; that the steatite of catalogues of engraved gems is for the most part serpentine, a harder mineral having an essentially different constitution; and that under the conventional term "plasma" several other minerals are included, such as jade and smaragdite, both varieties of hornblende, and even the beautiful rich green variety of serpentine known as antigorite.