Alright, so basically for this to work, you have to get your uncut gems through PvP, or low prices. What I am talking about so far has nothing to do with the trick, its just that if you buy uncut low, and sell cut high, then you should be getting gems from PvP or low prices.
For this example, we're going to use Bold Cardinal Ruby, and Runed Cardinal Ruby.
(The following prices are taken directly from my realm. And this whole idea was given to me yesterday when I did it to a person who needed something cut.)
So you cut some gems back when both types were at an equal price value. Now, undercutters have been bringing the prices of one type of the gems lower and lower.
You have:
Bold Cardinal Ruby x2 - and it sells for 105g.
Runed Cardinal Ruby x2 - and it sells for 135g.
Someone asks in trade for a JC to cut a 20 str gem, and they will give a 25g tip, you do that for them. They trade you the uncut gem and the gold. You trade back to them one of your precut Bold Cardinal Rubies. Once they are on their way, cut the Ruby they just gave to you into a Runed.
Bold Cardinal Ruby x1 - and it sells for 105g.
Runed Cardinal Ruby x3 - and it sells for 135g.
135g + 25g tip = 155g technical value for one of the Runed rather than a 105g value for the Bold.
So basically you just made 50 EXTRA gold for doing nothing.
It doesn't make sense when you have it in your head, but once you put it down on paper it works perfectly.