There are five areas you pass through in getting the jewel from the Diamond Mines. First, you will need a hardhat. You get this by completing the Safari. (see related question)
Beginning at the mine entrance :
Once you have all five jewels (green, purple, red, white, blue), return them to the totem in the museum.
The Sphinx is at Giza on Nabooti Island (Egypt is in north Africa). The blue jewel is in the tomb beneath the Sphinx.
The tomb is inside the Sphinx at Giza, on Nabooti Island. (see related question)
On Nabooti Island, the Blue Jewel is in the Mummy Room of the Sphinx at Giza.
It is the final challenge in the tomb under the Sphinx on Nabooti island. (for the solution, see the related question)
There are no keys on Nabooti Island. The Sphinx at Giza is opened by the Moonstone that Vince and the tomb robbers have found. It is placed in the ring atop the Sphinx. (see related question)
its what blocks you step on sweetie
Vince is Vince Graves, fortune hunter and tomb robber. He is looting the tomb of the Sphinx at Giza on Nabooti Island. He has the Moonstone in his bag, and if you get it, you can open the Sphinx. (see related question)
There is no known security code on Nabooti Island.
The "guards" are tomb robbers at the Great Sphinx at Giza. They will attack you unless you wear a disguise. Get the Blue Lily from Blue Nile Falls and swap it for a turban at Nabooti. This may fool them.
The Moonstone will open the Sphinx. Climb to the top of the Sphinx and put the stone in the ring there.
play a different island
The blue jewel for Nabooti is in the tomb inside the Sphinx at Giza. (see related question for solution)