"I thought it was a piece of paper or foil from a candy wrapper," she told park officials, the New York Daily News reported.
"Then, when I touched it, I thought it was a marble."It was a 3.850-carat canary diamond that park officials say could be worth $50,000 to $60,000.
Tana and her parents, from Oklahoma City, had been sifting through the park's volcanic soil for about two hours when she made her find on the surface.
Tana had been begging her parents to visit the park after a 12-year-old boy from North Carolina found a 5.16-carat diamond in July.
The park, the only diamond-producing site in the United States that is open to the public, is said to contain more than 75,000 canary diamonds, which have a distinct yellow colour.
Its "finders, keepers" policy" is drawing huge crowds and almost 400 diamonds have been dug up this year.
Few though are of the size and quality of the diamond Tana found, Joan Ellison, a spokeswoman for Arkansas State Parks, told NewsOK
Tana had fallen behind her parents when she found the rock.
"I was about to sprint to join my family, and God told me to slow down and look," Tana said. "Then, I found the diamond!"
Her mother Amanda Giordano Told NewsOK: "She screamed at me, 'Mama, mama I found something.'"
The rock Tana found is similar to a 4.21-carat rock discovered in 2006. That one was appraised for about $50,000 to $60,000, said Bill Henderson, assistant park superintendent.
"It appears to be of the same quality," Henderson told the New York Daily News.
"This particular yellow canary will knock your eyes out. The colour is so brilliant," he said. The value though is still unknown and Tana's parents have placed the diamond in a safety deposit box.
She said she may turn the diamond into a ring or use it to help pay for college.