Seal engraving is the art of sinking designs in intaglio on gems and hard stones. When the subjects are of an artistic kind the art is termed "gem engraving," and when a design is carved in relief it is called " cameo cutting." The tools and processes are similar in all three branches. The tools consist of small revolving wheels, the edges of which are charged with diamond dust, moistened with neat's-foot oil for hard stones, or with oil or water for soft stones, the polishing being effected with rottenstone and water. The object is held on a " cement stick," and is thus applied to -the lower edge of a wheel. The sapphire is cut slowly but smoothly: the ruby is cut slowly, being apt to break off in small pieces, leaving a rough edge; caruelian and bloodstone are of close structure, and may be cut slowly. The softer stones can be cut with greater rapidity, but the effect is not so smooth as with harder stones, the amethyst being as soft a stone as can be engraved smoothly. When such soft substances as glass or marble are engraved, the tools soon deteriorate, the diamond dust embedding in the work and thus re-acting on the tool.

The tools have long conical stems for fitting into the hollow mandril of a small foot-driven lathe-head, 'they are of iron wire, softened to take up the abrasive material easily, and around the stem of each tool is cast a tin or pewter plug that fits the lathe mandril. As the tools must run truly, they are fixed in the mandril and there turned. The general shape of the tools is that of a small disc more or less rounded on its edge, which is the cutting part, and which, for fine lines, is nearly a knife edge. For sinking large shields the tools are more rounded, and in some cases almost spherical. The rounded tool cuts more rapidly than one with a nearly fiat edge, and is chiefly used for removing the bulk of the material, while the flatter edge is used for smoothing the surface. To allow the tool to be applied to sunken flat surfaces without the stem interfering with its action, the edge is made conical. The tools are seldom larger than 1/16 .in diameter, and are sometimes as small as 1/150in., very small tools being made by wearing down on rough work. To prepare the diamond dust it is mixed with olive oil.

A small quantity is applied to the slowly moving tool; this is then moistened with some non-clogging oil, such as sperm or neat'sfoot. Stones to be engraved are often mounted on a handle about 5 in. long and 3/4 in. in diameter, the cement being coated with sealing-wax to prevent adhesion to the Angers. If the stone is set, its setting is inserted in a notch in cork or bamboo cane. The surface of a hard, polished stone is roughened by rubbing on a soft steel plate charged with a minute quantity of diamond dust and oil, or, if the stone is soft, on a leaden plate with fine flour emery. The outline is then carefully sketched in with a brass point or scriber, aud the surface within this outline is sunk. For dotting out an outline a small sharp-edged knife tool is used, a thicker tool with a rounded edge perfecting the outline; a still thicker tool is used for clearing out the material. The surface is finished with a smaller and flatter tool. Curved lines are more easily engraved than straight lines; and colour lines (or lines that show the stone surface between) are engraved with a tool having two knife-edges. The front edge cuts the required depth of line, while the second faintly marks out a parallel line; should the double-line tool tend to " run over," i.e. to overlap any previously cut outline, finish the lines with a single knife-edge tool.

The work is watched during the cutting through a lens mounted in an adjustable stand directly over the tool, the work being brushed from time to time. The engraver, however, depends much on the sense of feeling for placing the work in respect to the tool, and upon hearing for judging of the progress of the work. An impression of the work is occasionally taken in a black wax made by mixing fine charcoal powder with beeswax, and an impression of the finished stone may be taken by dusting it with vermilion, cleaning its surface, and theu pressing into hot red sealing-wax on a thin card or thick paper. So that the engraver's hands may be perfectly steady and free, he usually rests the palm of the left hand on the cap of the lathe headstock.while the forefinger and thumb embrace the revolving tool and grasp the upper end of the cement stick. The forefinger and thumb of the right hand grasp the stick just below, and the right elbow is supported on a small cushion on the bench. When the engraving is finished, polish is restored to the surface by rottenstone and water on a pewter lap. The engraved surfaces of seals are not usually polished, but those of gems are finished carefully with copper tools charged with fine diamond dust.

Boxwood tools with still finer diamond dust follow, then the copper tools with rotten-stone and water.