Kornerupine

Kornerupine was a rare fancy stone usually found in stream beds, eskers, or ridge-pit gravel.

Description
Kornerupine came in green or brown translucent crystals that were hard and yielded solid faceted gems up to medium size. A typical specimen had a base value of 100 gp.

Powers
The green variety of kornerupine had a most unusual magical ability to record sounds. When all but the last word of a magic mouth spell was cast upon one of these gemstones, the kornerupine became primed to record. It would remain in this state indefinitely until the caster touched the gem and spoke the last word of the incantation. Instead of a magic mouth appearing, the gemstone recorded all audible sounds within 20 ft (6.1 m) for the next four minutes. This audible archive was permanently stored in the gemstone and could not be removed, altered, or silenced without destroying the gem, but could be played back on demand an unlimited number of times. The value of the recording could drastically increase the value of a particular stone, be it a ballad by a famous bard, a solemn oath, words of wisdom or endearment from a deceased loved one, or conspiratorial evidence.

The brown variety of kornerupine had no known magical properties.