The robbery took place around 4 p.m. Sunday, KTXL-TV, Sacramento, reported.
Thieves broke into the vault at the museum, trying to get their hands on the Fricot Gold Nugget, a very rare crystalline gold nugget, but ended up leaving without it.
James Hutchings of the Roseville Rock Rollers said the gems and gold cannot be sold in their natural form on either the legitimate or black markets.
"They probably would take them to an oxyacetylene torch and just melt them down into blobs of gold, which would be an absolute travesty," Hutchings said.
The California State Mining and Mineral Museum does have a surveillance system and the video of the incident has been handed over to the California Highway Patrol, the investigating agency in this case.