The Crown of Horns a mystical Dragon item that Thorn seeks in Crown of Horns (Book). It was made out of crystal, as described by Fone Bone, in Crown of Horns.
The Crown of Horns was located in Tanen Gard, a secret burial ground of the Dragons. The Crown of Horns was one of the most well kept secrets that the Dragons had, and if it was found, the punishment was death.
The Crown of Horns is a large wall/column that is tinted a goldish color. Fone Bone mentioned that it was made out of crystal.