Karina Ille, 21, was spending the day with her boyfriend searching the elusive gem through piles of rocks that had been hauled out of the California State Gem Mine in San Benito County, the Los Angeles Times reported.
The mine's operators pull out the rocks and then charge visitors $70 each to spend the day looking for the gem.
If they find benitoite, visitors can wash the rocks, study them under black lights and take them home.
Ille said the piece she found turned blue under the black light.
"The guy who runs the place got freaked out when he saw it," she said.
Mine owner David Schreiner said the gem she found is extremely rare and likely worth a lot of money.
Ille said she planned to take the gem back home with her to Austria after their trip.
The most sought-after benitoite star is the six-pointed "star of David," which can be worth as much as $50,000.
About two dozen of them are known to exist, the Times reported.