What defines a hollowback cabochon?

A hollowback cabochon is characterized by a curved, undercut bottom designed to lighten the tone of the stone. This specific shape allows light to enhance the play of color and appearance of the stone from the top while reducing weight without sacrificing appearance or quality. The undercut creates a depth effect and can also help the stone appear more vibrant by facilitating better light transmission through the material.

The other options do not fully capture the defining feature of a hollowback cabochon. A flat surface on the bottom typically describes a different type of cabochon, while a smooth, polished top surface is a common trait of all cabochons, not exclusively a hollowback. An intricate design carved into the surface does not pertain to the hollowback characteristic and is more related to carved stones or specialized engraving. Understanding the unique design of a hollowback cabochon is essential in recognizing its visual enhancements and applications in jewelry making.