Of course, this makes all the more fascinating the show episode (S4E7) where Melisandre is taking a bath without her ruby choker. This scene was not in the books, many wondered if D&D had messed up, but I think the omission was quite deliberate.
The only person to see her like this is Stannis wife Selyse. Selyse has this strange expression the entire time. Like she cannot look directly at Melisandre yet also cannot look away. At first you think it is that of a religious "uptight" person seeing a naked and very sexual woman. But if you rewatch the scene , and "pretend" to yourself that Selyse is seeing Melisandre as she actually appears, it actually fits better. WE still see the seductress, Selyse sees the hag. She is horrified yet fascinated and not at all shocked. She knew all along (just like she knows the true fate of her daughter), Melisandre explains she does not need to use her "tricks" on the true convert (Stannis is NOT a true convert but his wife is) .
When Melisandre explains about her potions, and the potion that makes it so no man can resist her sexually, Selyse almost nods with relief. Did she use this on Stannis? (It would explain so much! she thinks as she stares in horror at the hags body, which still looks very alluring to the viewers.) "I didn't have to," Mel replies. Selyse actually shudders at this thought.
Anyway -- sorry if that was offtopic -- I think the ruby choker is what we might call a "foci" for Mel's magic, including her glamors but also much more. It purified the poison by via a magical means ("burn away").
Rewatch the scene where she takes her bath without the ruby choker, and this time, picture Selyse seeing the hag in its true form but still its obedient servant.
"If they are lies milady, aren't they best avoided?" Selyse refers to Melisandre's illusion of a sexual temptress. "Not always," Mel replies. When she discusses "the trick that led them to the truth," she is talking to Selyse as someone who has crossed over that line, into the true believer camp. She does not need to use ruby chokers with Selyse.
I think it's clear the missing choker in the bathe scene, was not an oversight but instead was a clue for us. Mel uses tricks with most, she uses none with Selyse. It also means that Selyse is not acting under any charms, any illusions, any "ruby choker" magic mind tricks, when she agrees to take her daughter to the Wall. She already knows the truth.
She has already seen the hag for what it is, and she thanks The Lord of Light for sending Melisandre to them.