Intarsia

What Is Intarsia?
(via Gem Cutting: A Lapidary’s Manual) John Sinkankas writes:

The ancient art of imbedding small bits of colored material in cement to cover and decorate walls, floors, and other surfaces is called mosaic. Mosaic work using gemstones is popular among amateur gem cutters since it is both decorative and utilitarian. Surplus gem material can be used to advantage in ordinary mosaics, while exceptionally fine mosaic projects can absorb better material that is too small or not quite good enough for cabochons or faceted gems.

A variation of classic mosaic work is called intarsia, or sometimes pietre dure, or Florentine mosaic work. In this style of mosaic, flat pieces of stone are fitted together as closely as possible to make designs and pictures, much as pieces of glass are joined to make stained glass windows. However, intarsia is never transparent; pieces are always cemented to a base of stone, wood or other hard material. Some kinds of intarsia are inlaid into stone panels, which are then flattened to a common level and polished.

The beauty and effectiveness of intarsia depend on the skill used in making designs and selecting various kinds of stone to cover areas of different color and textures. The Italian method of inlaying intarsia is to carve out flat-bottomed recesses in soft slate to receive thin slabs of gem material (cut based on a paper pattern). The edges of the recesses are carefully trimmed to make them square and to avoid unsightly chipping. Since black slate is used, it is not necessary to try for a very exact fit because any slight gaps will be filled with an asphaltic mastic cement that matches the slate color. A truly perfect polish is not practical with this method because the heat generated will make the mastic flow and allow sections of stone to shift. However, a well-moistened pitch lap should overcome these difficulties and allow application of an excellent natural polish.