Through Goblin's Gate
Transparent, cubical stone
Singing Stones appear in the Wormwrithings on an encounter roll of 5 or 6.
Except as an object of desire, they are useless. If a very loud sound (explosion, spell) occurs in the same area as two or more stones, they will begin a feedback harmonic. This grows from soft to very loud, and after the third minute, every minute deals 1dY*M damage to all creatures in earshot, where Y is the number of stones, and M is the consecutive minute, until physically restrained or moved out of earshot of each other. Each minute they have a M/10 chance of breaking.
Approximately fist-sized, naturally cubic or off-cubic crystals, Singing Stones are completely clear, but in torchlight will give a faint rainbow sheen, and double objects seen behind or through them. To Dwarves, or those who pass a DC 15 Wisdom check, they give off a faint harmonic hum, in fact, sonic reflections of the sound around them.