A glossary of terms regularly used in our industry.
A cabochon is a gemstone which has been shaped and polished, as opposed to faceted (explained below). The resulting form is usually a convex (rounded) obverse with a flat reverse. Cutting "en cabochon" (French, in the manner of a cabochon) is usually applied to opaque gems, while faceting is usually applied to transparent stones.
A faceted gemstone is one that has been cut, rather than rounded (diamonds, for example). Facets are flat faces on geometric shapes. Transparent gemstones commonly have facets cut into them in order to improve their appearance by allowing them to reflect light and displaying the underlying symmetry of the crystal structure.
Lustre is the way light interacts with the surface of a crystal, rock, or mineral.