There is no such thing as an impossible DC since a natural 20 succeeds all the time. Setting an "impossible" DC just means you can only win on a natural 20.
Doesn't matter, passing with a 20 still gets you a fail outcome. But what the OP is missing is that these dice checks are suposed to give a gameplay extension of trying to do something seemingly impossible and then failing, which is why a certain NPC comes bail you out and explains why your strategy with certain jewels is no longer enough.
As far as complaints go about the game, this is a pretty clueless one.
