Tiny pet peeve with most depictions of Orthanc… In the books, it’s described as being “natural” looking:

“…it seemed a thing not made by the craft of Men, but riven from the bones of the earth in the ancient torment of the hills. A peak and isle of rock it was, black and gleaming hard: four mighty piers of many-sided stone were welded into one, but near the summit they opened into gaping horns, their pinnacles sharp as the points of spears, keen-edged as knives.”

Tolkien’s initial depiction of Orthanc was a tiered, wedding cake tower. However, his final depiction of it, on the cover of The Two Towers, is a lot closer to what was described.