enamel—the sum of the effects due to lustre, 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 transÂlucent 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 oi
serpentine known as antigorite.
