The result is usually an unhappy customer. Two factors are most important in determining value. Rarity and beauty. Rarity has to do with supply and demand. Common gems will cost less than rare ones.
Beauty causes demand.
All things being equal, the more beautiful gem will always command a higher price. A more intense color and a better cut will give a gem more beauty and thus a higher price. Two one carat sapphires may be very different in value. The one that is a stunning, brilliant, lively blue will command a much higher price than the one that is inky, dark, and dull. If both gems are just as beautiful, the larger one will be more valuable than the smaller one.