--- spoilers after this---
I noticed, that when you go to the volcano and pick up the hammer thing, that is used to break the egg, and go to the edge where egg can be placed before you've prepared it, the game allows you to put the egg down, but not to hammer it broken. This leads into a bug, that won't crash the game but won't allow for you to move either. Only thing you can do is to restore previous game.
Funnily enough, if you don't have the hammer you can't even place the egg on the ground.

Yeah, I noticed that too. Drove me insane until I located the Altar.