She was wearing a silver ring on her index finger; it was plain, just silver without any jewels on it. However, its width was a little more than your normal rings and covered her whole proximal phalanx.
This was a magic item that was used for wireless communication. When charged with mana, the miniature runes inscribed on the ring—which were so small that they looked like the circuits on a circuit board—converted the user's mana into electromagnetic waves and then emits them on a custom wavelength that can be set by the user to have a private channel.
However, since the technology isn't perfected and is still in development, there are some flaws in it and the electromagnetic waves die out after a while, and you can only communicate via the rings within a hundred-meter radius.