Based on simple-looking white-gold bands, 8 of these rings were created centuries ago for the use in the higher courts of a long forgotten empire. Their enchantment is simple: the wearer can not lie while wearing such a ring. It is possible to refuse to answer or to be evasive, but nothing the wearer knows to be false can be conveyed either by word or deed (e.g., shaking of the head; winking etc.).
Any of the rings can be removed at any time; they are not cursed although some would count them as such.
A character under the influence of an undetectable lie spell must make a roll against the rings' magic resistance of 50%, based on the level of the caster of the spell once when the ring is donned. So a spell from a 7th level caster has only a 10% chance of defeating the ring. This roll applies for the duration of the spell even if the ring is removed and put back on and the wearer has no way of knowing whether it has worked until they first attempt to lie.
Monks of 9th level and above have a chance to defeat the rings as if they were charm spells.
Finally, the rings can be automatically defeated by a Wisdom score of 23 or higher.
The high gp sale value reflects the desirability of these rings to governments and rulers.