The Virgin of the Rocks or The Madonna of the Rocks is a term used to describe one of two different paintings. They are extremely similar in name, subject matter, and appearance, but there are important differences between the two. The difference between the paintings is one of the plot points in The Da Vinci Code. To reduce confusion, below they are broken down below by the museum in which they currently reside:
This is a painting generally accredited to Leonardo Da Vinci c. 1508. It was stolen from Florence during the Napoleonic wars and recovered in a small town in Austria some 50 years afterwards. Since then, it has been bought by a French merchant before being sold to the National Gallery in London, England in the 19th century...
Official site from the National Gallery: [1]
An almost identical painting with the name Madonna of the Rocks is to be found in the Louvre, however it is not as universally accepted as being Da Vinci's work. French authorities claim predominantly that the work is authentic, due to the fine brush work and use of chiaroscuro, or contrast between light and dark, characteristic of many of Leonardo's works.
Official page from the Louvre: [2]
Note
There are separate accounts of which of the 'of the Rocks' paintings are verifiably Leonardo Da Vinci's. Some can be found online attesting to the precise opposite above. The history of both is muddled, especially by their reference in 'The Da Vinci Code' which may have had an effect on common opinion about the history of these paintings.
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