[Lost Dutchman]

First of all, the stone is Aswan granite, not Black Basalt.

Second, it would have been quite easy for the Arabs to decipher the Hieroglyphs over 1000 years ago because they were still in use until 1400 years ago.

[DWPittelli]

Since 1822 it had been thought that Champollion was the first person to break the hieroglyphic code, but a recent analysis... has proved that Arabic scholars decoded hieroglyphs 1,000 year earlier.

I wondered how the Egyptians built such a large civilisation when they couldn't read their own writing!

On a more serious note, you are right about the rock type, but most people thought it "black basalt" until it was cleaned by the British Museum recently:

"Analysis of the stone has shown that it is a granite-like rock and not a basalt as it has previously been described.... All of these contaminants, including the white paint, were removed to reveal a grey stone with a natural sparkle and a broad pinkish vein at the top."

http://www.thebritishmuseum.ac.uk/conservation/cleaning2.html