The robbery, which took place on Friday in broad daylight, involved two men wearing hoods and armed with pickaxes who threatened two workers at the California Mining and Minerals Museum in Mariposa, the California Highway Patrol told reporters.
Multiple news outlets are reporting that while the thieves did make it out with a hefty load of gold, they didn't get away with the most valuable item in in the museum— the nearly 14-pound (6.35-kilogram) Fricot Nugget, a giant crystalline gold mass unearthed in the Gold Rush era.
According to reports, during their attempt to grab the massive nugget, the robbers triggered an alarm that alerted authorities who swarmed the museum but were unable to capture the thieves.
Authorities said the unique pieces that were taken would be easily identified, which could make it difficult for the robbers to sell them.
"It is uncommon for most citizens to possess such minerals," the CHP said in a statement.
While no suspects have been identified at this point, the two museum employees who were present during the crime were not injured, but remained shaken from the experience, the California Department of Parks and Recreation, which operates the facility, said in a news release.
As a result of the incident, the museum is closed while workers repair display cases and other items damaged by the robbers. An inventory of the stolen items is also currently being conducted by the parks department, the Christian Science Monitor reports.
Reports indicate that this was the second heist this year of rare, valuable metals in Northern California. In February, thieves made off with large chunks of gold that were on display in a Siskiyou County courthouse.
Investigators told reporters they were trying to determine if there is a connection between the two heists.
The museum, which is home to more than 13,000 artifacts, was previously on a state list of facilities being considered for closure to help save money.