What exactly is the "Oily Black Stone", and why is it everywhere?

Question

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
• Yeen
• The Toad on the Isle of Toads
• The Five Forts of Yi Ti
• Hightower's foundations
• Asshai

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?

Answer 1

Well according the ASOIAF wiki entry on "black stone," it is used in many supernatural instances.

For example, Maester Theron theorizes that the black stone was produced by the Deep Ones - "[a] queer, misshapen race of half men sired by creatures of the salt seas upon human women."

Meanwhile, "The Bloodstone Emperor rejected the traditional gods of Yi Ti and instead encouraged the worship of a black stone said to have fallen from the sky."

Based upon what I found, I would venture to guess that it does not have anything to do with "dragonglass." As to why it is found in such "diverse locations," it may just be a coincidence or it may have something to do with the aforementioned supernatural elements present in the series.