This tension set ring looks like it started life as a regular shaped ring and was twisted with a big pair of pliers.  It's actually machined that way out of solid billet, complete with its unique squared back, which keeps it from rotating on your finger.  The outside is smooth and flowing and grips a medium sized round stone.  The stone shown is around 5.5mm in diameter.  Works well alone or with its matching companion ring.

*Note: due to the asymmetric profile, engraving should be kept short to work best.