There is no such thing as stable currency, value is all relative, a scientific unit of value only exists in people's imagination. People tends to believe that their domestic currency is the unit of value, but not any foreign currency. So for a Japanese people, USD is not stable at all since it rose a lot against Japanese Yen recent years. Similarly, it rise against Rubble and petroleum by 50% in one year, while it fell against bitcoin by 1000% in two years