A man in South Texas found out that a painting he inherited from his mother was an original work by famed Mexican artist Diego Rivera worth $1 million. 

Rue Ferguson says his great-grandparents bought the painting by Rivera in the 1920s. He got it last May when his mother died. He showed it to appraisers when the PBS program came to Corpus Christi to tape three episodes for its 17th season that starts in January.

The program's account executive Mariel MacNaughton tells the Corpus Christi Caller-Times it's exciting to come up with such a gem. The painting is the highest appraisal so far for the upcoming season.

Ferguson had kept it in a box in his office. It's now in a bank vault.

"I was dumbfounded. I didn't know what to say," Ferguson told the paper. "I thought it might be worth a tenth of what they said. I had no idea."

The painting, "El Albanil," is one of Rivera's oldest works. It was completed in 1904, when Rivera was just a teenager, the newspaper said. 

Rivera is one of Mexico's most prominent artists and was the husband of famed artist Frida Kahlo. His frescos have become iconic works of art that highlighted the struggles of Mexico's peasants as well as the disparity between the titans of industry and the working class. 

Besides Mexico, Rivera's frescos are in many U.S. cities including New York, Detroit and San Francisco. His work was also featured last year in New York's Museum of Modern Art. 

Based on reporting by the Associated Press.

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