What are the possibilities in a mystery chest?

So after being deprived of key fragments for 2 weeks, I have finally earned my 1 last key fragment needed to make a hextech key. And with full excitement, I busted open one OUT OF the 27 chests I had earned with such ease (sometimes, it was too easy that some games I didn't even deserve it). The loot gloriously opens slowly, with every frame of animation set upon it, it reaches to its climatic point only to give me 1 shard. 1 SHARD ladies and gentlemen, 1 shard! And you want to know what it was? ONE shard. By this point you'd think that was a joke, it should have given me at least a summoner icon, but no it was just a Lissandra shard.

So after reading my beautiful summary of what just happened in 15 seconds of this game, I'm curious as to what are the possible combination this mystery chest really offers???

(I'd love one of the riot members to actually explain this one to me XD)