I thing that it's a Dragon egg. It's a bit similar with snake egg (the shell of this egg is made of dragon's skin as well as snake eggs are made of skinny substance). Also It's possible that this mine was closed because of dragon's visit. And this dragon put his (her) egg into this rock as in the place which contain a lot of gold (and dragons like gold).
P.S. In Belarusian Language dragon will be "Цмок"(Tsmok).