Anything can be a currency so long as there is a value to it. Your seashells, your stones can be a currency if people are willing to put a value to it. You are confused with the difference between a legal tender and a currency. There are no laws within most jurisdictions that prohibits Bitcoin to be used as a currency. It is purely up to the merchants and the consumer preference.
Yes, everything can be used as currency or in other words as mean of exchange, if people in that community accept to use it like this. Value of a thing depends on how people assess it and accept it.
The history of Stone money in the Yap island.
One of the amazing facts about the stone money, or Rai as they are called in Yapese, is that these gigantic stone discs were not quarried on the island.
Being legally, legal tender will help its legality status but even does not reach to this level, people can use bitcoin as a mean of exchange many things in society. They of course have to consent with lawyers or read legal regulation documents by themselves, to avoid legal problems.