The gnome does appear randomly on your farm and will offer a blind trade.
Most of the time he wants crystals, but he also takes gold if you’re lucky!

The rewards can be: items, amulets (no relics!), magic chips, gold or crystals.

The trade gets more expensive with your level, but the reward will get better too!

LevelCrystalsGold
1-5110
6-10920
11-1514149
16-2020199
21-3029699
31-40392 499
41-50496 499
51-60759 999
61-709912 998
71-8011914 998
81-9014916 998