The Riddle of the Stone
Many of the descriptions of the Stone do not suggest a real object at all. The stone itself is a puzzle to be solved.
"It is made of fire and water;
it is a stone but not a stone;
it comes from God but does not come from God;
it grows from flesh and blood, and yet is made of animal, vegetable and mineral;
unknown and yet known to everyone, it is generally considered worthless and yet supremely valuable."