Dyes extracted from natural materials such as the bark of trees, flowers, leaves and minerals are known as natural dyes. Vegetable dyes are a sub-category of natural dyes, referring to color that come from plant matter only. Mordants, which are usually minerals, are used to fixed dyes on cotton. All natural dyes generally yield lasting colour though some colour are sensitive to sunlight. The most common natural dyes are found in plants, but certain insects produce a red dye and certain shellfish produce a purple dye. Rust is an ancient mineral dye.