I have to agree with this. Diversify- One thing I do myself (among many others, but this is just the example) is transmute epic gems every week, and even if they are the same color of gems I don't just cut them all into the same thing- for example if I'm doing Kings Ambers I will cut some Quick, some Brilliant, maybe a hit rating one in there if I got lucky procs and feel like I'd be listing too many of one kind. Same with meta gems- I spent some time watching prices and seeing which ones sold at all for a decent profit, and I just cut a few of those and auction them and when they're gone I cut a few more- I never list more than 5 at a time (this number is going to vary on your server depending on how populous it is), and mostly I don't get too many back.

In addition to diversifying, it really pays to spend some time getting to know the markets. Don't just buy a whole crapload of Cardinal Rubies with your honor because you know that's the most popular/most expensive color; look around, watch the market. Are there regularly a ton of unsold gems of that color because there's so much competition? I mean, on my server, something like a purified dreadstone that is needed by a LOT of casters to activate their meta gem is down to like, 80 gold at times. I avoid that cut like the plague as a result.

Getting to know the market also really helps because you'll be able to tell when a price is really much lower than it should be- and what the high threshold would be. Plus, continuing to watch it will help you know when it's time to stop buying low/selling high because the price isn't going that high anymore. I was making quite a bit buying ebonweave cheap and selling it high for a while, but now the price has stabilized more in the 30g a pop stage (I was buying around 20 selling at 50g per), so I don't bother. Which brings me to another point- get to know the prices so you know what is a very low price and what isn't; Set your thresholds and stick to them. It helps.