Any stones you associate with Tyr? And is it possible to bless an oath ring alone?
For the oath ring: Yes, I believe so. The important part of an oath ring is the oath more than the ring I think. And if the oath you’re swearing is to a god then as far as i’m concerned the only beings that need to be present are the god and yourself.
For stones/minerals I associate with Tyr (not exhaustive): Actinolite, Sapphire, Youngite, Agate, Bellingerite, Huntite, Magnetite, Rutilated Quartz, Diamond, Andalusite, Tiger Iron, Anatase, Spinel (specifically, black), Acmite, Rumanite.