In The World of Ice and Fire (TWOIAF), a companion book to the A Song of Ice and Fire series, there are repeated references to mysterious "black stone" (often described as oily or greasy), which composes, among other things:
- The Seastone Chair
- The Toad on the Isle of Toads
- The Five Forts of Yi Ti
- Hightower's foundations
What exactly is the significance of this black stone? Is it related to dragonglass? Why is it found in such diverse locations? Have Elio, Linda, or GRRM (the authors of TWOIAF) said anything regarding it?