The stone of folly or the stone of madness refers to a hypothetical procedure in the 15th century involving trepanation

Trepanation

Trepanning, also known as trephination, trephining or making a burr hole, is a surgical intervention in which a hole is drilled or scraped into the human skull, exposing the dura mater in order to treat health problems related to intracranial diseases. It may also refer to any "burr" hole created...

 and extraction of a stone, thought to be the cause of the patient's madness. This procedure is demonstrated in the painting The Extraction of the Stone of Madness

The Extraction of the Stone of Madness (The Cure of Folly)

Cutting the Stone, also called The Extraction of the Stone of Madness or The Cure of Folly, is a painting by Hieronymus Bosch in the Museo del Prado in Madrid, completed around 1494 or later....

 by Hieronymus Bosch. The procedure and the painting depicting it inspired the 2002 short film of the same name.