Fire-bearing stones which were found in the east and which were either male or female. When they are far from each other the fire within does not ignite, but when they are brought together they will at once burst into flames, burning everything around them.
Hence is was advised that nuns and monks should be kept separate, lest the flames of their lust be kindled.
- Philippe de Thaon. Le Bestiaire, 1395-1405.