I will tell you later, im just doing it.

You must go to the tomb of Mar.
When you see water, go into the water, use your jetboard.
Go to the middle of the pool and take the left path, follow the path, go past all obstacles and at the end, you must stand on a switch, it will unlock a beam.
The second beam will be unlocked when you pass the second test of manhood.
The two beams will open a gate that leads to the stone.

Tim van der tuuk is right but ill let you on a little secret Jaks name is not Jak its Mar and he has a dad you will see in the third Jak or should i say Mar3