what is the ancient tomb.

fly to egypt and ask one of them about it

somewhere near fortree city near where you get the devon scope from steven

go to pacifilog and surf to the left on the current and it will take you to a place where you can dive go down and read the braille then you will be in the tomb

The braille messages read:
Emerald
Those who inheirit our will, shine in the middle
Ruby + Sapphire
With new time, hope and love, aim to the sky in the middle.

Both versions mean that you have to HMFly in the middle of the room to get Registeel.

After solving the sealed chamber puzzle (read the post about pacifidlog and the
currents)follow route 120 until u see a ledge, jump down, go to the wood bridge, go across
and go to the large rock surrounded my 6 other rocks and there should be a cave with
braille in it.

Emerald:
Flash at the middle braille message

Ruby + sapphire
Fly at the middly braille message