It depends on the broker (every broker is having its own set of symbols) - for example:
Currency price of the country with the most exports and imports of silver and gold.
The other way is to use correlation:
Forum on trading, automated trading systems and testing trading strategies
Indicators: MFCS Currency Correlation Chart
Sergey Golubev, 2014.02.04 09:27
Australian Dollar Strongly Correlated to Gold, Silver, Steel Prices (based on this article)
View forex correlations to the SPDR Gold ETF Trust
(GLD), United States Oil Fund ETF (USO), SPDR Dow Jones Industrial
Average ETF Trust (DIA), UK FTSE 100 Index, and IShares Silver Trust ETF
For example (if you check the images above): Gold and AUD/USD are having almost perfect correlation on W1 timeframe.Silver is in negative correlation with USD/CAD for example.And so on ... (you can check it by yourself).