But they didn't get away with the biggest prize of all -- the nearly 14-pound Fricot Nugget, a giant crystalline gold mass unearthed in the Gold Rush era.
During their attempt to grab the rock, the robbers triggered an alarm that alerted authorities who swarmed the museum but were unable to nab the thieves.
At least two robbers wearing hoods and armed with pickaxes threatened workers during the heist Friday at the California Mining and Minerals Museum in Mariposa, the California Highway Patrol said.
No suspects have been identified.
The heist was more bad news for the beleaguered museum, which is home to more than 13,000 artifacts. It is operated by the California Department of Parks and Recreation, and was previously on a state list of facilities being considered for closure to help save money.
Authorities said the unique pieces 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.
It 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 were trying to determine if there is a connection between the two heists.
Roy Stearns, a spokesman for the parks department, would not comment and referred all questions to the CHP.
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