Coir is a coarse fiber extracted from the fibrous outer shell of a coconut. The individual fiber cells are narrow and hollow, with thick walls made of cellulose. They are pale when immature but later become hardened and yellowed as a layer of lignin is deposited on their walls. There are two varieties of coir. White coir is harvested from the coconuts before they are ripe. The fibers are white or light brown in color and are smooth and fine. They are generally spun to make yarn that is used in mats or rope. Brown coir is harvested from fully ripened coconuts. It is thicker and stronger and has greater abrasion resistance than white coir. It is typically used in mats, brushes, and sacking. The coir fiber is relatively waterproof and is one of the few natural fibers resistant to damage by salt water. Both modulus and tensile strength depend strongly on the type of fiber (bundle or single filament) and moisture content.