You will find Oculi all over the place, mostly in treasure chests. They are gemstones you can equip to your party, with different benefits according to their colour and size.
Press Options and go to the Oculi screen, then click on the big "Craft Oculi" button. You can arrange up to 3 gems in the slots on the left, and combine them into a different gem. If you press square it will show you what kind of combinations are valid.
- 3 gems (same colour + size) = 1 larger gem
- 2 primary colour gems (red/green/blue) = 1 secondary colour gem (orange/purple/turquoise)
- 3 primary colour gems (same size, different colours) = 1 white gem
- 3 secondary colour gems (same size, different colours) = 1 black gem
- 1 white gem + 1 black gem (same size) = 1 rainbow gem
For the trophy, just make any valid combination - eg. a red gem and a blue gem, or 3 small red gems.