Probably the mean gold-silver price ratio, since with the current levels of inflation the prices need to be higher for both over time just to keep the value.
And that is not accounting for factors that increase prices even more than just keeping up with inflation, such as increased industrial demand for silver or the increased stockpiling of gold by central banks.
On the other side of the equation, the prices of both seem to have been suppressed for a long time - probably to make fiat currencies and the financial system look more stable than they are. Especially a large rise in the price of gold has always been interpreted as signifying financial and currency instability.
