I found this one while browsing Guitar Blog , a fertile source for lovers of weird and bizarre guitars. This is a one-off concrete bodied guitar made by Parker Sloan. He made the body himself, by casting the body in a mold. The neck and other parts are from Warmoth. The whole thing weighs 7.5 lbs. Parker is pleased with the tone, a unique sound with surpirsing resonance.
To build the guitar, Sloan cast a mold of an original guitar body by using a liquid rubber compound that sets up once exposed to air. Using his rubber mold he experimented with several different types of concrete until he was satisfied he had body that was both light and strong enough. He added the hardware and bolted on a neck. You can see him playing the US national anthem on another concete guitar (a concrete S.G.) in the video below.
I seem to remember some granite guitars in the seventies. The sustain was incredible but so was the weight: they had to be played on stands. CNTarasi makes guitars with corian tops; corian is a type of artifical stone.