I don't worry about the value of gems. We (me and my gamers) are not shoppers. They deduct a percentage of overall wealth, and I tell them what kinds of stuff are available wherever they are at.
If they needed it as a plot point, then it depends on what they are trying to exchange and how much the other person wants gemstones. I play it by ear - I never use an exact exchange rate. I treat it much like how I would do it if they were trying to barter away a book, a castle, a slave, or a piece of information; what is the exchange rate of a fief or manor, for example? The seller talks it up, and the buyer talks up whatever it is he has to trade.
Barter is, almost by definition, not an exact science. I worked for a roofing company as their accountant for a while. The owner of the roofing company was building his own house, but he didn't want to take out loans or anything - so he bartered for his house, piece by piece. He wanted a basement, so he found a guy with a backhoe and offered to re-roof his house if he would dig out a basement. He then re-roofed a lumber company in exchange for lumber. He did the same thing for plumbing, flooring, construction, windows, etc. Did all of those exchanges technically come out even? No. But he got what he wanted out of what he had.
I simply do not bother with exchange rates since they would vary too much from place to place and from time to time - and, like bjorntfh says, it is doubtful anyone has that much cash laying around. I am, by training, an accountant with a lot of specialty in economics, so I would be tempted to mess with supply, demand, elasticity and a bunch of other variables, but it is easier and more fun to just treat gemstones like I would if they were trying to barter away information. They could trade high-value stones for a noble title or a fief, perhaps, but probably not for coins.
But if the value is needed for shopping, our group doesn't worry about it. We don't roleplay trips to market. No one in our group would be interested in that - they are too interested in the adventure, not in the mundane.