Bank of Russia expanded its gold reserves almost sixfold since the mid-2000s, creating the world's fifth-biggest stockpile that's valued at about $140 billion. It's the type of asset it could sell to shore up the ruble, which has plunged as global economies isolate Russia following its invasion of Ukraine.

Doing so will be difficult. Sanctions forbid U.S., U.K. and European Union institutions from doing business with Russia's central bank. Traders and banks are wary of buying the country's bullion indirectly or using other currencies out of fear of reputational damage or falling foul of penalties. And senators in Washington want secondary sanctions on anyone buying or selling Russian gold.

"This is why they bought their gold, it was for a situation just like this," said Fergal O'Connor, a lecturer at Cork University Business School. "But if no one will trade it with you, it doesn't matter."

Moscow may need to look east to central banks in nations like India or China to sell gold or secure loans using it, according to CPM Group Managing Partner Jeff Christian, who has followed precious metals since the 1970s.