If the hermit is captured, you can return to the island where you found the hermit to activate it. Diamonds have a chance to appear when the cave explodes, or you can open the box behind the second island. However, the total number of diamonds is limited, so try not to let the hermit be caught, otherwise you will not unlock a lot of things in the later period.
Solution to the hermit being captured
If the hermit is captured, you can return to the island where you found the hermit to activate it. Diamonds have a chance to appear when the cave explodes, or you can open the box behind the second island.