Using a ring mandrel to stretch a bezel can make a bezel very slightly cone shaped, which may not work well with some stones, such as this straight sided druzy.
This method will not distort the shape of the bezel. Thread a metal jump ring mandrel or a dapping punch handle through the bezel. With your hands on both sides of the mandrel, roll the mandrel with the bezel back and forth across a bench block a time or two, using a bit of pressure on the mandrel. This will stretch the bezel out just a bit. Reshape the bezel, check the fit, and repeat the process if needed. Works every time!