Intaglio designs are carved below the surface level of the stone. This leaves a smooth face with no projections. Another term for it is "reverse carved." The carving can be done from the top, or from the under side of the stone.
Some intaglio designs are cut so that there is an illusion that there is slight "refief" or height to the carving, but the carving always remains below the surface. This type of intaglio is known as cavo rilievo, or "Sunk relief."
To visualize intaglio, think of it as the opposite of a cameo, where the design actually protrudes from the surface as a sort of sculpture.
Originally, intaglio designs were carved into shells, marble, quartz, and other hard stones. Glass is often a medium used. Designs are often floral with many having roses as the main feature. Other often seen designs are those of people in profile, knights and other human designs.
Here are some examples of intaglio jewelry:
My favorite form of vintage jewelry is a cameo, but intaglio designs are a close second. You can find the last two pieces for sale on my Vintage Jewelry Lane Website.