Rare earth elements are not exchange-traded in the same way that precious (for instance, gold and silver) or non-ferrous metals (such as nickel, tin, copper, and aluminium) are. Instead, they are sold on the private market. This makes their prices hard to monitor and track. However, prices are published periodically on websites such as mineralprices.com. The 17 elements are not usually sold in their pure form. They are usually distributed in mixtures of varying purity, e.g. "Neodymium metal ≥ 99.5%". because of this, pricing can vary based on the quantity and quality required by the end user.