Cameo cutting, or the engraving of gems in relief, is effected with the same apparatus, and by the same general methods as those employed in engraving corresponding forms in intaglio, and both arts are occasionally practised by the same individuals. The principal differences in the manipulations of the seal engraver and the cameo cutter arise from the design being in the former case wrought concave, and in the latter convex. The tools with which the former are produced, being themselves convex, they may in most cases be selected of counterpart curvatures to the concave details required in intaglio engraving; but the convex forms in cameo cutting, have to be produced with convex tools, which cannot therefore be selected of counterpart forms, but the convex surfaces have to be produced by twisting the stone about at all angles beneath the rounded edge of the tool. For this reason the engraving of gems in relief, is usually considered to be more difficult than engraving in intaglio. On the other hand, however, the deep recesses in cameos are generally more accessible than those in intaglio, and the principal source of difficulty in gem engraving is therefore in some measure avoided.

The stones selected for engraving in cameo, are generally those called onyxes consisting of two layers of different colours forming a strong contrast, as the black and white layers of the agate, or the red and white layers of the carnelian. The design is almost always engraved exclusively in the white layer, and the dark coloured layer forms the back ground, the contrast of the two colours serving to render the design more distinct. Sometimes onyx stones having three or more layers of colours are employed for cameos, these are selected when either from the great amount of relief desired in the engraving, the thickness of the white layer would be insufficient to allow of the entire design being engraved in it, or that it is desired to make the most prominent parts of the design of different colours in order to improve the effect.

Mineralogists generally restrict the name onyx to a variety of chalcedony, consisting of alternate layers of brown and opake white, but those artists who work in precious stones usually attach a much more extended signification to the name, and the following interesting particulars from the pen of Mr. H. Weigall will explain the cause of these discrepancies.

"All the stones in different coloured layers employed for cameos, are known to practical men by the general name of onyxes; but some confusion has arisen with regard to the nomenclature of stones of this class, in consequence of the imperfect information of those authors who have undertaken to describe them. It is a remarkable fact that no author who has undertaken to describe the onyx, has given this simple, and to all practical persons, intelligible, description of it, namely, a stratified stone occurring in any of the semi-transparent or opaque varieties; thus there is the onyx of the sard, called the sardonyx, that of the carnelian called the carnelian onyx, and so on through the whole variety of stones.

"The name onyx is derived from a Greek word which signifies nail, and the authors before referred to, have evidently been perplexed to make out any resemblance between such an object, and that particular variety of the onyx which they happened to describe. Thus Pliny could see no resemblance to a human nail in the specimen from which he took his description of the onyx (which appears to have been a bad sardonyx), and he therefore thought it must be a horn or hoof, and fancied a resemblance to a horse's hoof. Theophrastus seems to have described a cloudy specimen of the carnelian as the onyx, and he fancies it resembles the pink and white colours sometimes observable on the human nail."

Mr. H. Weigall however suggests that there was an original propriety in the name, and that it most probably arose from the practice of the ancients in staining their nails, for if the stain were only applied at distant intervals of time, the lower portion of the nail would grow between the applications, and present a band of white at the bottom of the coloured nail, and thus render it a fair type of the onyx stone.

Mr. Weigall has made inquiries of travellers who have visited those Eastern nations where the practice of staining the nails is still continued, and has found this view to be corroborated, as they agree in stating that the nails commonly present two colours exactly resembling an onyx.

The stones to be cut into cameos are prepared by the lapidary, and to avoid wasting the material, each stone is left as large as possible. The cameo cutter has therefore to select a stone as nearly as he can in accordance with his intended design, which must be afterwards modified in some degree to suit the stone.

As a preliminary step to cutting the cameo, it is most important that the artist should have a clear conception both of the design, and the capabilities of the stone. To assist in this, he first makes a sketch of the design on an enlarged scale, and then having considered the degree of relief, that will be adapted to the thickness of the white layer, he makes a model in wax of the exact size of the stone.

With unimportant works this is frequently omitted by practised artists, who depend upon their skill for overcoming any difficulties that may arise, but it is at all times a great assistance in elaborate works, especially to those who have not great practice. The model and stone are carefully compared, and any alterations that may be demanded by the formation of the stone, are first made in the model.

When the stone is in three layers, additional care is required to adapt the design to the stone. It is at all times desirable that the line of division between the colours of the two layers forming the ground and figure should be distinctly defined, but it is sometimes an advantage when the transition between the two colours in the upper layers is more gradual. For instance, in cutting the head of a Medusa, in a carnelian having one layer of white between two of red, if the lines of division between both the layers of red and the white were sharply defined, the features must be cut entirely out of the white layer, and the upper layer of red must be reserved for the snakes, but if the transition between the upper layer of red and the white were gradual, a faint tinge of colour might be left on the cheek with great advantage to the effect, and the skilful engraver of cameos will thus avail himself of every opportunity for heightening the effect that is offered by the formation of the stone. When the stone consists of several layers of colour, considerable scope is afforded for the exercise of the judgment, in selecting a design in which the whole of the colours can be rendered available.

When the design has been accommodated to the stone as nearly as possible, the outline is sketched on the surface, and cut in with a knife-edged tool, and the superabundant portions of the white layer beyond the outline are removed down to the dark layer forming the ground. The general contour of the figure is next formed, and this is followed by the principal details, which are sketched and cut in succession, care being taken to preserve sufficient material at the most prominent parts, and to advance the engraving uniformly, so that the general effect may be compared, from time to time, with that of the wax model.

The surface of the back ground is conveniently flattened with the broad flat surface of a tool such as fig. 1201, and the difficulty of removing the little irregularities on the rounded surfaces of the figure, with the convex edge of a revolving tool, may be entirely avoided by the use of a tool called a spade, consisting of a piece of soft iron about 3 or 4 inches long, the end of which is filed at an angle of 45 degrees, and charged with diamond powder. The spade is held in the fingers like a pencil, and rubbed with short strokes, either straight or circular, to reduce the irregularities of the surface. The last delicate touches are executed with very small tools, and the cameo is finally smoothed and polished in the same manner as the best works in intaglio.

The method of carving cameos in conch shell, described on pages 1094 to 1097 of this volume, is more expeditious, and presents much less difficulty, than the engraving of cameos on gems, but the shell cameos do not admit of the delicate cutting and elaborate finish usually bestowed on true cameos, and they are also much less durable.