It is a man-made polymer, with a backbone of silicon and oxygen. Silicon is an element found in silica, i.e., sand, one of the most common materials on earth. However, to make silicone, silicon is extracted from silica (it rarely exists by itself in nature) and passed through hydrocarbons to create a new polymer with an inorganic silicon-oxygen backbone and carbon-based side groups. What that means is that while the silicone might come from a relatively benign and plentiful resource like sand, the hydrocarbons in silicone come from fossil sources like petroleum and natural gas. So silicone is a kind of hybrid material.
Reference: Plastic-Free: How I Kicked the Plastic Habit and How You Can Too, p. 277)