In my opinion, certain bonuses or unique traits (UTs) can be useful in providing civilizations with extra resources.
This can be done through bonuses such as the Polish Stone mining bonus, the Portuguese Forager bonus, Burgundian Vineyards, and Paper Money.
These bonuses can provide a small but steady stream of resources to a civilization.
For instance, I think it could be interesting to have a bonus that would allow Gold Miners to generate Stone, albeit in a very limited capacity.
The ratio of Stone to Gold would need to be very low to ensure that Stone remains a precious resource.
A ratio of 1 Stone per 12 Gold mined (about 67 Stone per 800 tile of Gold) could be appropriate.