@Horizon:
If he had a Devil's fruit, the WG would have definitely used them. That's why the were created in the first place. Why would they let the most dangerous pirate on the planet walk around with complete usage of his DF? That would be completely stupid on their part.
We can't know if kairoseki was as present at the time of Roger's execution as it is now. Vegapunk is probably the one who has discovered how to manipulate it and make objects out of it. The stuff with marine ship undercoating at least hints that. There might have been an unprocessed piece of kairoseki somewhere on his body, or something.
So it can't be a deffinitive proof that he never had a devil fruit. I myself started suspecting he had a devil fruit after seeing what Whitebeard, whom he tied with was capable of. That and Oda carefully hiding anything that would resemble a fight from Roger flashbacks.
Mind you, I'm not saying it has to be Gomu Gomu, but it wouldn't be the first time we were left in the dark by the characters, even though "someone should have mentioned it".
Btw, just realized you can't see what Roger's shackles look like anywhere in the manga. Those wooden ones from anime took over my memory. If someone can remember if we ever saw them in manga, please…