Tiny flecks of metal can be added to a rosin recipe to produce a unique tonal property when applied.
Gold flakes can create a soft and smooth sound and are useful on steel strings or for softening the sound of an instrument with an overly bright or harsh tone. Silver is also sometimes added which creates a concentrated sound for softer instruments or string types (such as gut strings), or copper which helps to make a velvety smooth sound and can be useful for beginners.