Fleur de sel, literally “flower of salt,” meaning the finest and most delicate, is a special product of the sea-salt beds of west-central France. It consists of the crystals that form and accumulate at the surface of the salt pans when the humidity and breezes are right; they’re gently raked off the surface before they have a chance to fall below the surface, where the ordinary gray sea salt accumulates. Fleur de sel forms delicate flakes, doesn’t carry the particles of sediment that darken and dull the gray salt, but is said to carry traces of algae and other materials that contribute a characteristic aroma. This is possible, since the interface between water and air is where aroma molecules and other fatty materials would concentrate; but to date the aroma of sea salts has not been much studied. Thanks to the labor required to make it, fleur de sel is expensive, and is used as a last-minute condiment rather than as a cooking salt.