That is true, no money can be stable, the fiat are not even stable, that is why people are misleaded that fiat pegged coins are stable coins when fiat itself are not stable, the centralized authorities will always control fiat price.  Not to talk of metals, valuable materials and cryptocurrencies like bitcoin that their price is affected by demand, supply and fiat decreasing value, their price will be volatile because of these reasons, bitcoin price can never be stanle, nothing existing with the so called instrinsic value that can have a stable price, nothing have a stable price, it is not possible.