Thank you very much for all your kind compliments on our game! Knowing that Grove can come across as both very enjoyable AND unique means a lot to us, as we definitely strived hard to push RPG Maker in ways we've never quite seen done before.
As for your missing chest, you'll need to use a save to return to the mine full of bats. When you get to the area with the pink crystals, a bat will close a gate and lock you in. Go and talk to the bat and accept his offer.
Thanks for wanting to play our game through to completion, and for the very kind review!