Here's an eBay listing for John Denver's Ovation Adamas 12-String (Item #: 110489680149)
According to the seller, this Ovation Adamas guitar was used by John Denver from 1979 to 1983 on records, world tours and television appearances.
This is an early Adamas 1 model. Introduced in 1976, the Adamas was produced by the Kaman Corporation as a flagship model to their popular Ovation range and was the first acoustic guitar to be produced with a carbon-fiber and birch veneer composite top.
Charles Kaman also owned Kaman Aerospace which was producing, among other things, helicopter rotor blades. And it was Charles who was inspired to incorporate carbon-fiber aerospace technology into a musical instrument for the first time. Good enough for helicopters, good enough for guitars.
And in his own quiet way, John Denver inspired a whole generation of guitar pickers. And even if you didn't dig his stuff, you were at least aware of it -- Denver was a huge star who was all over the airwaves in the '70's. Chances are that it was this particular Ovation Adamas you were listening to.
But don't take my word for it -- do the necessary research before plonking down the moola!