Muenster, Germany - German geologists said Thursday they have discovered in India some of the world's rarest rock, dating back to the birth of the planet when the Earth was covered with a hot ocean of melted stone.

The fragment from the primeval crust is only the second ever discovered, said scientists at the University of Muenster.

The ancient magma formed more than 4 million years ago as the planet slowly cooled in the Hadean period. The fragment, found in Orissa state, yields answers about what the Earth was like in those times.

The find was detailed in this week's issue of the journal Nature.

The only other piece of early magma, which was located in Canada, has been dated at 4.3 million years old.

Normally, old rock is sucked back into the ground by the churning of tectonic movement and melted again, but the finds show some pieces of the old crust are still extant.

Internet: http://dx.doi.org/10.1038/nature08089 (dpa)