The pattern for a mosaic is sometimes referred to as a cartoon because it is just an outline with no attempt at shading or color. The purpose of the cartoon is merely to map out the major work lines and color fields, and so the cartoon is relatively simple, even for photorealistic work. The texture and color mottling and finer details come from the tile.
Simplified Yet Exact
The cartoon is not required to have all the lines needed to create an image, but what lines it does have should be exact. For example, you shouldn’t try to draw all the flecks of color in the iris of an eye, but you should have that iris drawn exactly where it goes.
Many people are drawn to the idea of making mosaics from marble and stone, mostly because that was the material used by the ancient Romans but also because they would like to make a mosaic from natural materials in subdued colors.
Nevertheless, as soon as these people start trying to source materials, they quickly become frustrated with how limited the color palette is in marble mosaic, and they usually end up mixing the stone with smalti or ceramic or porcelain tiles, or they use dyed stone or synthetic stone for certain colors.
In either case, the mosaic usually doesn’t have the look and feel that was desired, which is really a tragedy because superior results could have been more easily and cheaply accomplished had the artist used all glass and merely restricted the color palette to more subtle hues.
Before you convince yourself you need to work in stone, spend some time looking at glass mosaics made from subdued color pallets.