Yes, fat grafting for buttock augmentation does require there to be a donor site with adequate fat. If the person is naturally lean, this probably wouldn't be an option for them.
But for anyone else, the patient can get one, the other, or both.
Here's a surgeon who does it all:
http://www.betterbuttocks.com/As for the material, it's just a solid silicone implant. As far as I know, breast implants are the only implants to be filled with either saline or liquid/gel silicone. And 'solid' doesn't translate to "hard as a rock".
Master "Dr Roberts would appear to have done some stellar work
" Dragonfly