Answer:
Silicone and rubber are often used interchangeably. They have a similar chemical makeup, but technically silicone is a "rubber-like" material. Natural rubber is a substance that comes from trees, so technically silicone bracelets are a "man made" form of rubber and don't have true rubber in them. I guess it just depends on how technical you want to get. You can check out Wikipedia for some details or talk to an expert who manufactures them, like MonkeyBands.com.