Norman,
I too have suffered this problem.
With due respect, when I normally choose an item in the design menu "buy mode" I need to tap an empty space to 'place' the item at which point the purchase is confirmed.
This is not the case with 'coins for gems' (and I also presume wiith 'Magic Shell' / 'Magic Box').
Given the efforts that players have gone to in aquiring 10 gems I hope that you can pursude the powers that be at TeamLava that a small tweak here would help pacify what I suspect is, or will be an ever inceasing moan from players.