Modern times. Modern world.
In my story it's possible for several countries (let's say USA and Russia) to basically get gold out of nothing (their prime cost to do so is <1% of current market price and can be made lower, but is not zero).
It's not possible for other countries to get access to this technology by any means.
USA declare that now they now support gold standard again and freely exchange dollars for gold bullion. Russia thinks about doing so but in future and start make. Russia and USA have to coordinate policies on this issue on at least some level.
Same process can be used to produce other precious metals / gems.
Consequences to world economy in short term? Especially consequences to other countries
A lot of people (especially in Arab countries / China) say that dollar is not backed by something of value and it's better to checkange . Now it is backed.
Very small number of people knew that supply is essentially unlimited / how exactly this work. Even them can't get 'a little for themselves'. World at large just knew that U.S. Department of the Treasury announced that Federal Reserve will happen (Federal Reserve got to perform actual exchange and store gold).
assume that REAL source of gold is direct energy-to-matter conversion + essentially unlimited energy source. 'Gold standard' idea was suggested by one of USA advisors to who have direct control over process.
Assume that even when others will find out about whole thing, they can't dublicate it themselves. Just can't.
Assume that produced gold can be made indistinguishable from natural if needed (same isotopic composition).