How do I respond to my mother who thinks her things have been replaced?

This has been going on for a couple years, but up until today, it was small things like clothing, shoes, jewelry. Generally, she thinks we have taken xyz, and replaced it with a different item. Part of the issue is her eyesight (she's mostly blind but can see big things), another part is her failing memory, and a major part is delusions. This morning, the item was an antique wheeled desk chair (heavy). She thinks we took her desk chair and replaced it with a different one, and gave hers to someone not related to us. I took her to look at the chair; there's a matching non-wheeled chair, and they both match an antique desk (all have been refinished). She tried telling me that she can't always be wrong. I didn't argue, and tried to say very little about it, but she started getting nasty to me, verbally. I know I can't convince her (it never works), but I'm also not going to say that I took her chair and replaced it. When it was just clothes that she did this with, she'd leave them out, and I would just make them disappear to the basement. She won't wear anything that she thinks isn't hers. Inserting obvious logic doesn't work (but I don't expect it to). This happened with a ring the other day, and it was OVERNIGHT, so there's no way anyone could have gone to a store to get a replacement, and of course, she insisted it wasn't hers. Suggestions?