A local coin dealer will hide historic currency worth a total of $50,000 in its third annual citywide challenge on April 25 The 1851 $50 gold slug was one of the first coins minted in San Francisco. Seth Chandler / Witter Coin You know what they say: If you find a gold octagonal $50 slug from 1851, pick it up. All day long you’ll have good luck. This weekend, people in San Francisco will have the rare opportunity to find such a coin—worth about $25,000—during Witter Coin ’s annual scavenger hunt. The local numismatic dealer will hide 10 coins across the city, valued together at more than $50,000. The hunt is a nod to San Francisco’s prospecting legacy. The city’s history is inextricably tied to the California Gold Rush , the mining craze that followed the 1848 discovery of gold in the nearby Sierra Nevada mountains. “This city was built around the pursuit of gold,” Seth Chandler, owner and CEO of Witter Coin, says in a statement . “We wanted to create something that brings that spirit back.…