About $2 million worth of gold nuggets and precious gems were stolen Friday afternoon from the Californian Mariposa Mining and Mineral Museum, a State Parks spokesman said.
According to CBS47 News, a group of armed robbers threatened museum staff before stealing the gems and gold, but no one was hurt.
It was reported than some of the stolen gold pieces are fairly large, about the size of a fist in some cases, so police think they would be very difficult to sell.
The Mariposa museum, visited by more than 20,000 people per year, contains examples of rare and valuable minerals from around California and the world. Parts of the collection date back more than a century.