Flutes can be made of various materials, including metal, wood, and plastic. The most common materials used to make flutes are copper-nickel, silver, gold, and grenadilla, which is a type of wood. The quality of the material used to make a flute affects its resonance, timbre, durability, weight, feel, and price. Beginner flutes are often made of nickel and plated with silver, while intermediate to professional flutes are made of solid silver, gold, and sometimes even platinum. Wooden flutes are also available, and they are usually made of African Blackwood, granadilla, boxwood, mopani, cocobolo, or couswood. The tube of a wooden flute is made of wood, while the keys and all their mechanisms are made of either silver or gold. The pads attached to the surface of the keys to cover the holes are made of cork and felt, and the springs that provide tension to hold the keys firmly against the holes may be made of steel, bronze, or gold.