The non-linear pricing of different sizes (weights) of diamonds means that it is not realistic to exchange, for example, 2 quarter carats (50 mg) for 1 half carat (100 mg). With commodities such as gold, it is clear that 1 twenty gram bar is worth the same as 2 ten gram bars, assuming the same quality. In most terminal markets, there needs to be a readily available standard quality, or limited number of qualities, available in sufficient quantity to be tradable. This is a major factor which affects liquidity. The large number of variables in diamond quality make commodity-like pricing difficult.