Since a lot of people have asked me about this, here's a list of all the jewelry locations in the Barn.

Don't read this if you want to figure it out yourself, obviously.

Two of the jewels are obtained automatically. For the others:

1. In the northwestern pumpkin patch, behind the wall of vines. You need to direct the waterer to water the pumpkin patch to get through.
2. In the third screen of the carrot patch. Pull the carrots and defeat the monsters.
3. Defeat the random encounter in the cornfield. It can occur on any screen that doesn't contain a predetermined treasure or event.