Trivia: The end credits of the film don't have the usual "No animals were harmed during the filming..." Maybe this is because when the Little Mule 4WD is being chased, it clearly runs over a chicken.
Trivia: The film was written by Diane Thomas. Thomas was a struggling waitress and actress, and had been working odd jobs to make ends meet when she wrote the script. While there is some dispute over how it happened, eventually the script made its way to actor Michael Douglas, who championed Thomas and helped get the movie made. The script was so well-liked, she ended up getting numerous offers, and at one point was even attached to help write one of the "Indiana Jones" sequels. Sadly, she was killed in a car accident only a year after this movie's release, and it remains the only film she was credited for writing.
Trivia: The studio was so sure the film would be a flop, they preemptively fired director Robert Zemekis from his next project, "Cocoon," and instead gave the job to Ron Howard. Ironically, the film ended up being one of the top-ten grossing films of the year.
Answer: Any emerald over 1 carat in that beautiful cut and condition would probably go for over 250 million. Real emeralds over 5 carats are rare and the price goes up exponentially after 2 or 3 carats. 305,000 per carat after 5. So yeah that one was probably like 800 or more carats.