There's a lot of precrafts around level 18 that sell for gems.
Here's a list of a few that I sold for gems in the past:
Flawless Hawks (level 16 axe)
Flawless Acidic Splash (level 17 potion)
Great and Flawless Jade Amulet (level 20 pendant)
Great Nordic Lute (level 22 instrument)
Great and Flawless Sabaton (level 15 boots)
There are more, but that's a good place to start. There's a few tricks that can help speed up the process if you want to go this route.
1) You can craft different types of items to use all your resources. If you only craft hawks, you will run out of iron. However, if you craft hawks in half of your slots and acidic splashes in half, then you can continually craft (or closer to it).
2) When you need to fuse up the item to the correct quality, you can combine it with another item to half the fusing time. Since they have about the same value, you lose nothing and you can fuse more of them faster. For example, instead of two good hawks to make a great hawk, use a good hawk and a good acidic splash to make a great item (either a great hawk or a great acidic splash). The same applies for common to good and great to flawless.
3) You can sell them instantly for gems in the requests, but with a little bit of patience, you can sell them through offers for a few more gems. I made the mistake of selling instantly when I first started gathering gems like this.
Because gems are worth a lot more than items in that level range (levels 1 - 25 or so), that's also one of the better ways to make money.