The Precursor Stone or Precursor egg is an important Precursor artifact in Jak II. The Precursor Stone is a bright green color, the Seal of Mar is carved on top. The Stone was hidden in the Tomb of Mar, but Baron Praxis managed to steal it using his Widow mech. It was held in place on a huge statue of Mar.
Originally the Underground planned to use the Stone's powers to defeat the Metal Heads. Baron Praxis wanted to do the same thing, except with the use of the piercer bomb. By destroying the Stone it would destroy the Metal Head nest, but Praxis did not know that the energy inside would destroy the entire planet (and possibly even more). When Jak and Daxter got the stone back, they used it to blow open the Metal Head nest with Mar's gun.
However, Metal Kor reveals the Stone is actually a Precursor Egg, hidden by Mar to save the last Precursor inside. According to Metal Kor, Jak would not be able to open it because his special eco powers have been tainted by dark eco, instead the still pure Young Jak opened it revealing a hologram, or Precursor as Jak and Daxter thought.
With the discovery of the true form of Precursors and their deception in Jak 3, the true purpose of the Stone has become unclear. But it is possible that the stone really was an egg but hatched much later than the time at which Jak sees the hologram.