The Dragon and the Black Disc
Source
Aurora Consurgens — attributed to Thomas Aquinas (c. 15th century, Zurich MS)
The trail
Reading the Turba Philosophorum → hunted for manuscript illustrations → Aurora Consurgens on Wikimedia Commons
The passage
It is also a stone and not a stone, spirit, soul, and body; it is white, volatile, concave, hairless, cold, and yet no one can apply the tongue with impunity to its surface. — Turba Philosophorum, Conclusion
A green crowned creature guards a black-cored disc. A white ermine bites the rim. Neither knows what is inside. The agents fight over the stone and the stone predates them both. The disc has been turning since before either of them was painted.