Re: umm... major question!
Kidney stones are alot bigger. I heard about something like this on that late-night radio show 'Strictly Sex with Doctor Drew'
First of all, penises can't sneeze XD
If it just came out like that, then it's probably because it's just some left over in your tubes. It sometimes kinda clumps up in there, and it can just come out at any moment (although it might not clump up).
If it dries hard then that's fine. It shouldn't dry up really fast, and if it's as hard as a rock then that's a bit strange. But if you haven't noticed anything else weird, like pains or lumps, and it doesn't happen again/on a regular basis, then you should be just fine!