No matter what obstacles you remove, the number of Gems drop from them will always follow the sequence above.
We can get new obstacle after every 8 hours, It means there are total 21 obstacles respawn on your land per week. So you can get up to 45 Gems every week. Normally, we have a Gem Box (25 Gems) every 2 weeks. It means you can earn up to 45×4 + 25×2 = 230 Gems every month.