You need to be in the right place at the right time to find a dungeon with golden or big gems. Each dungeon in Attack mode can only be attacked a limited number of times and a rare thief will pass by without at least trying their luck. This means that such a dungeon is only available for attack for a short time, and in this case, "Finders keepers, ..." You can try to change the time of the game, perhaps most players are online at the same time and then your chances of finding big gems are greatly reduced. 

So you'll need a lot of luck - and we're keeping our fingers crossed for you!