Pure gold can be mixed/alloyed with many other metals. The purer the gold content, the more expensive the cost. In the UK, gold fineness is officially tested at one of the assay offices and given a valid mark of fineness. 18 carat gold is hallmarked 750, which means there are 750 parts of pure gold per 1000. This mark will appear alongside the maker’s mark and the hallmarking office’s official stamp. A number mark alone does not guarantee fPure gold can be mixed/alloyed with many other metals. The purer the gold content, the more expensive the cost. In the UK, gold fineness is officially tested at one of the assay offices and given a valid mark of fineness. 18 carat gold is hallmarked 750, which means there are 750 parts of pure gold per 1000. This mark will appear alongside the maker’s mark and the hallmarking office’s official stamp. A number mark alone does not guarantee fineness.ineness.