I was thumbing through the Summer 2006 issue of The FretBoard Journal (Number 2), a gorgeous magazine for stringed instrument players, collectors, and builders, and came across this short piece about "tone balls." These are the "nebulous balls formed from the bits of lint, dust, hair and insect husks that fall into the soundholes of guitars and mandolins."
Steve Olson, who repairs guitars at Elderly Instruments in Michigan has been collecting tone balls for years and has "catalogued dozens of examples by make, model and year of the host instrument." He says his favorite tone balls are the "densely compressed, perfect spheres formed by rolling around under the cone of old National guitars (top left)."
The Fretboard Journal Number 2 is sold out at the publisher's website, but is available at Elderly Instruments' website for $9.95 (the same issue contains an article I wrote about a ukuele strumming robot used to break in newly-made ukuleles).
Read more in Music at Boing Boing