The silicones are polysiloxanes, i.e., inorganic polymers with a backbone of alternating silicon and oxygen atoms.
Silicone can be produced in a wide range of hardnesses (durometers). Subdermal implants are typically made from silicone of durometers 10-30, while O-rings are usually around 60 durometer.
There are many different grades of silicone, from those designed for industrial uses, right through to purer forms used for short and long term medical implants. For subdermal implants, only silicone suitable for long term implantation should be used. Silicone is becoming more and more popular as a substitute for PTFE in subdermal implants due to it being much softer than PTFE at low durometers. The main concern with implanting silicone is that the surface is very tacky and can easily pick up contaminants, so it needs to be washed and sterilized properly and handled with care.