What is the unit in which value is measured? There is no SI unit for value, and it seems unlikely that it would ever be possible to define one - it would basically require very surprising breakthroughs in psychology.
Hence people do the pragmatic thing and measure value in the currency that is predominantly used in their life. They use USD or EUR or whatever as their unit of value.
And in that sense, the value of gold and silver is clearly not constant, as a sibling comment explains.