The hard ass cement bolt on ones are actually more likely to be saline.
Saline implants come to the doctor empty adn they are insterted into the breast and then filled. The problem with implants is that if you don't fill them enough, they are nice and soft but you tend to see more wrinkling and they look awful.
If you fill them too much on the other hand, you minimize the wrinkles but they become more firm. Most doctors tend to overfill implnats slightly. Mine are 350cc sacs but are filled to 400cc.
Silicone implants as far as I can recall are pre-filled so there is not as many instances of "hard ass" silicone ones unless there is a secondary problem such as capsular contracture which is a whole other story.
Another thing to keep in mind is that in smaller chested women the implant is usually placed under the muscle. So depending what she is doing at the time you touch her boob her muscle may be flexed which would make it feel hard. I know if I flex my pecs I can make my chest ROCK hard lol, but relaxed my breasts are relatively soft.
