Taking the control of the Gems markets

We all know than usually trying to take control of a market without being 100% sure of how it works is a mad things to do, and it's still very dangerous if you're 100% sure... If you fail, you'll loose huge amounts of money, since it's a business that asks you to spend more the more bad are you getting at it.
There are lots of things that can go wrong: you may have underestimated the influx of the items you want to control on the market, or the stupidity of people just selling their things at the price they're used to, missing out the profit they would have had just by not opposing you.
That said, there are still some niche market that can be easily controlled.
In this days in which Cataclysm is still so new, this is true for most of the cut gems.
If you scan the AH you'll see that there are lots of cut gems which are crafted just by one or two players, and in very small amounts. Some of this players are very shy when it comes to ask for a good price, and their gems are quite cheap compared to other of the same color/utility.
So, this is my advice: look for gems at low prices (on my server this means 100-150g) and that aren't totally worthless (i.e. +40 Spirit), made by few people and in small quantity, buy them and re-list them for a hundred golds more... I never do this if there are more than 15-20 gems selling (and if they really are 15-20 I do it just when I know the auctioning behavior of their sellers) and I wouldn't advice you to do it with more than 5-10 gem of the same kind at a time.
If after some hours you've been undercut by a small amount of gold/silver/copper you can just undercut back as you would normally do, if they undercut you by a huge amount it's your choice between continuing to buy and re-list or get out of that market before you get hurt... You're the only one who can know what's better, since this depends both on your server, your gold and how much the other player can be adamant about its price of choice.
Anyway, since there are few players with that precise cut, most of the times won't be too difficult to impose your price, and you'll start to gain also from the cut you can't make.

By the way, on my server there are some strange people selling nearly everything for half its price (two days ago I found 10x[Shadowspirit Diamond] for 150g each, when they're going for 330g on my server's AH*).
I'm not sure why is this and if this could be happening also on your server, but you should try to write down names of people selling under-priced items and check for them.

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*And before you ask: it wasn't a stolen account he's still making things like this, if it was stolen Blizzard would have blocked him by now