In bezel setting, the stone is held inside a raised metal ‘tube’ that has roughly the same shape of the gemstone itself. The stonesetter sinks the stone down into the metal tube by removing metal from the inner wall. After that, the outside wall of the tube is bent or hammered over the stone.
The ‘tube’ is only really a cilinder if the stone itself is round. The cross section will be oval, square, heart-shaped… depending on the shape of the stone. This is called a ‘bezel’.
The gemstone is quite well protected in this style of setting, because it is surrounded by metal on all sides. The top of the stone still sits above the metal, so the stone still catches a lot of light and is still very visible. That top part is still exposed to knocks and abrasion however. If the material is too delicate for even bezel setting, flush setting offers even more protection but less of the stone is visible.
In properly executed bezel setting, the rim of metal around the stone will have an even width around the complete circumference. The metal should be completely flat and free of dents. It must touch the stone on the complete circumference of the stone. If it doesn’t, dirt will accumulate in the spaces. These will show as little dark dots (not unlike specs of rust) in the metal rim near the stone.