Base bullion is a term that refers to precious metals that are more impure and of lower quality than standard bullion. Synonyms for the term could include terms like "crude bullion," "inferior bullion," or "impure bullion." Other phrases that might be used to describe similar materials could include something like "rough metal," "substandard gold," or "poor quality silver." Some people might also use diction like "unrefined gold" or "unprocessed silver" as ways to refer to this type of material. Ultimately, the precise synonym used might depend on the context and the specific connotations that a person is trying to convey.