A collar is not made of leather. It isn’t made from metal either. It doesn’t have to fit around your neck. It can be anything at all, because a collar is an idea. A concept—a symbol of commitment in a relationship where one person submits their will to another.
Of course, we’re speaking specifically of the kind of collar a submissive wears. There are other kinds of collars and they can be nothing more than a clothing accessory—but with the right context it can be everything.
I’ve had more than a few collars in my life. Some I bought myself to wear because I liked the look and it’s always fun to wear an obviously “BDSM” collar around your neck, especially when you go to a fetish event so everyone knows you’re a submissive. For a while they were in style and you could get them everywhere in velvet, satin, or even leather, and when you walked around in a public place and saw someone wearing one you got to wonder if they were a submissive, or if they just thought it was cute. Maybe they saw yours and wondered too.