Sigring
The Sigring is the collar. Worn at the throat and never removed, it bears the owner's Oathbond sigil, which is the whole of its function: it says who she belongs to for anyone who looks. It also carries his name and address, which is what returns her if she is lost. A Sigring with no sigil means an unclaimed slave, and that is a matter for a magistrate.
Materials vary enormously and mean nothing about status. A field slave's is plain iron; a companion in a wealthy house wears worked silver; in The Iron Coves it is worn as ornament and matched to what the household wears. A magistrate does not distinguish between any two slaves and there is no rank inside the category. What the metal tells you is what work she does and what her household can afford.
Built to come off
Every Sigring opens. A clockwork release at the back works under two hands, without a key, so that any free person nearby can manage it and someone who has never seen one can work it out inside a minute; physicians are taught it as a matter of course. The clasp is also made to fail under load, so a slave who catches her collar, or falls, or is hung by it comes free rather than dying. A clasp that has done its job is replaced within the day and the collar kept, and nobody thinks anything of it. Forgegate makes the most precise mechanisms; every city with metalworkers makes its own.
The maker
A Kolaerth makes collars and holds an absolute confidentiality Oathbond, because a man who can make a convincing collar bearing any sigil is a man whose gossip would be worth a great deal to forgers. Ordinary work is cut fresh from a pattern the household brings; regular clients of standing have a die kept in the maker's safe, and everyone in the trade knows which shops those are. A new owner fits a new collar. What happens to the old one is the previous owner's business.
The bar
Some collars carry a black iron bar hung from the ring. It means one thing — this one is dangerous — and it follows the person, not the collar. See Male Slaves#The bar.