There are many other metals (of varying scarcity) just as there can be many other crypto-currencies. Both Bitcoin and Gold are the most popular of each type.

There are many other metals (of varying scarcity) just as there can be many other crypto-currencies. Both Bitcoin and Gold are the most popular of each type. Bitcoin can be forked and has been many times but the fork with the most proof-of-work is generally considered the 'one true Bitcoin'. This proof-of-work (aka longest chain) metric provides a way to measure the relative scarcity of various cryptocurrencies.