mitsu

This had me 'stumped' for ages! But I can help... I'm unsure if you have played 'golden sun-book one' before, but in that game, you play as four different characters... Which you will meet up with later in the game (sorry for the spoiler... I won't say where... But it's part of the answer that I tell you)

In the original game, if you played it, the main guy (known as isaac) got a gem called 'the orb of force' that allowed him to use the psynergy 'force', which is the psynergy you will need to use on this stump...

But there's a catch (and also why I had to tell you the spoiler) when you play golden sun 1, it gives you game data or you can link up two gameboys and transfer your data from goden sun 1 to golden sun 2...so that when you meet up with isaac, garet, ivan, and mia... They will be at the exact levels, holding all the djinni you found, plus they will be holding all the armour and items they had at the end of golden sun 1... This includes psynergy items... Like the orb of force.

But if you don't transfer these details by the time you meet up with isaac and friends, you will meet them at a generic level holding generic items... And no orb of force... So therefore you will not be able to get what's inside the chest (a masamune... Which is a sword) there are a few other items that you get in GS1 they wont automatically be holding in GS2 when you meet them.

On this very site, in the cheats, or kints section for this very game (or perhaps for GS1) someone has put one of the codes I am taking about (if you don't have one of your own) you can input it now by going to your start screen and selecting 'update'
I warn you though... To get the good items, including the orb of force, you need to put the gold code in which is extremely long... BUT I have personally tried this code myself, and while it doesn't actually give you all the fantastic items that the person has claimed it would... It does give you a fair lot... It's a pretty good code. Plus you will get the orb of force... And the masamune.

Cheers!