When watering a Crystalised Crop there is a chance for the teffressence gem to drop.
If this occurs, the gem will be picked up by the player and appear in the HUD as such, but the gem will not appear in the players gems list.
Steps to Reproduce
Water a gem plant until it drops a teffressence gem
Check gem inventory after picking up the gem
A developer has marked this issue as fixed. This means that the issue has been addressed in the current development build and will likely be in the next update.