Well, you could really balance it however you want. There are just a couple practical issues to keep in mind.
In the real world, pretty much every society wanted gold. It's shiny and pretty, and all that. So, no one questioned that gold coins had value. Even when societies switched to a more abstract paper-currency system, those bills were still backed by hard currency, somewhere. (It's only in the past century or so that we gave that up.)
So first, what gives these items of currency their value? Metal coins have value because worked, pure metals were ALWAYS valuable due to the time and effort of extraction, forging, etc., and that if need be the metals could be used for other things. For the others, this just doesn't work; if pearls are the sole currency for your Water cultures, why be anything other than an oyster farmer? If obsidian is the sole commodity, then anyone who travels to a volcano becomes rich. Commodities in general require their source to be rare/dangerous/distant to be of value. In the middle ages in Europe, pepper was worth its weight in gold, literally, because it was so difficult to get a large supply that far away from the source, and because as a grown product the amount being produced was limited. This isn't true of obsidian or quartz.
Second, there's the issue of cross-culture valuation. How much is a pearl worth to your Earth creatures, for instance? Would they value it at all? And if the currency is something available all over the place (like your quartz idea), this could cause problems. An Earth race who digs up a ton of quartz would destroy the Air races' economy.
There are two ways I can see this working:
1> The "currency" system. If the currency is actually WORKED materials, something more complex than the typical peasant could make. This could be a government thing; in the later Roman Empire, stamped gold coins could only be made by the Emperor; sure, other people had gold, but only the solidus coin had the set value merchants could trade in. This could work for obsidian, but not as easily for pearls. This is almost the same as if you use the currency simply as a representation, backed by hard currency somewhere else. So, if 1 pearl = 4 gp, it's because some government or bank has 4 gp stashed away for each pearl it issues.
2> The "commodity" system. If every civilization officially uses some widely-accepted currency (gold, silver, etc.), but internally uses a more relaxed system by trading a commodity. This works because the commodity being traded (like the pearls) holds a relatively constant value in its own right.
I'll give an example of what I mean. I play GuildWars, which has a simple 1 pp = 1000 gp currency system. High-level players who want the cool-looking armor all need two rare ingredients: Ectoplasms and Obsidian Shards. These items are only found in two places, and are pretty rare, so they tend to cost about 10pp and 4pp each, respectively, and the prices have stayed constant for the past month or so since players are using them to make the armors at the same rate they enter the economy. Now, when someone sells an expensive item in-game, it's become accepted that people can pay at least partially in Ectos at a 10:1 ratio, since the game doesn't allow cash trades of over 100pp. So, even though the system is officially a coin-based system, everyone has accepted that a commodity that holds its value constant is an acceptable substitute, especially since many of those players would have used the cash to buy Ectos anyway.
So anyway, to sum up what I think you should do:
Earth: Metals are fine.
Fire: Obsidian as a commodity doesn't work, because of the huge easily-available supply. But finely-carved obsidian coins would work.
Water: Pearls can be used as an internally-traded commodity. Outside of the Water races, though, only a few groups would value them nearly as much.
Air: For this, I'd actually suggest going to a pure barter system for day-to-day stuff, and possibly have them use everyone else's currencies for external trading.
But if you want to ignore all this and just pick an exchange rate for each, you can use whatever you want.
