Halite, commonly known as rock salt, is the mineral form of sodium chloride (NaCl). Halite forms isometric crystals. The mineral is typically colourless or white, but may also be light blue, dark blue, purple, pink, red, orange, yellow or grey depending on the amount and type of impurities. Sylvite is potassium chloride (KCl) in natural mineral form. It forms crystals in the isometric system very similar to halite. The two are, in fact, isomorphous. Sylvite is colourless to white with shades of yellow and red due to inclusions. It has a Mohs hardness of 2.5 and a specific gravity of 1.99. Sylvite has a salty taste with a distinct bitterness. Sylvite is one of the last evaporite minerals to precipitate out of solution. As such, it is only found in very dry saline areas. Its principal use is as a potassium fertilizer.