Potomac Crystal Navettes - Black Diamond Opal. The crystal navette is a machine cut, pointed oval marquise shape with many facets that will sparkle brilliantly in your designs. The navette is held in a 2 hole silver plated setting that allows you to create more easily with this beautiful stone. The Potomac Crystal Navette will work with any designs calling for a Swarovski 4228/S shape. They measure 5x10mm and come in a pack of 12.