I'm leanign towards just treating gems as money traded at same value. The people who would trade such gems would be jewelers, moneychangers, or rich people like lords. The jeweler is going to use it and so won't have it to sell two weeks later unless you ant to buy the 5,000 GP goblet that it is now a part of. If he is holding onto gems that he hasn't used, he'll probalby charge a 10% fee like the moneychanger will in both the case of buying and selling the gem. Rich people will either change for 10% like a money lender or even free as they may be in the market for such items. Then again, lords and such can make friends and influence people by preforming such money changing without a fee. In cases where the gem is actually used to pay for something, such as the manufacture of a magic item, it goes across like money at 100% value.
I figure that gem prices are somewhat regulated just as coin exchange rates are either by law or tradition. While there is a demand for gems as luxury items, I figure the more major demand is from the rich for a stable, non-volitile commodity that is easier to transport than gold, ie gems and precious stones. The demand for gems as a money item is greater than the demand for gems as a luxury item and stableizes the price so that other market factors don't change the price enough to worry about.