Diorite is one of the hardest rocks known to be difficult to work; its hardness is between 7 and 8 on the Mohs scale. However, due to its consistency, diorite is even more difficult to work than granite. Today, granite can only be worked with diamonds (hardness 7).
So how could the diorite statue of Pharaoh Kefren have been made in the 21st century BC with such precision that no imperfections from the tools used were visible?
At that time, the hardest metal known was copper or its alloys. There was no iron (hardness 4 on the Mohs scale), let alone low-carbon steel (hardness 5).
Iron did not appear in the East until the 12th century BC, and it did not appear in Europe until the 8th century BC. It is not known when iron was first used in Africa.
Even with iron, diorite would not be workable, as iron is much softer than diorite.
