LADIES AND GENTLEMEN, TIME FOR MORE controversy, brought to you by the one and only yours truly.
According to Qiu Chuji, finger-writing on stone is out of the question for Wang Chongyang. Yideng might not be able to write on stone either (actually it says he might not be able to write on wood, so I'm being rather lenient).
In DGSD, Jiumozhi's Duoluo Finger was described to be able to penetrate gold and stone: 穿金破石 was the exact expression used to describe this style.
Is it a fair assumption that Yideng would not be able to penetrate stone if he can't write words on this kind of surface?