They can come in different colors, they are both synthetically created and they have a weird history of State intervention.
For instance, the Wikipedia entry on Margarine is fairly enlightening regarding regulations promoted by the established Dairy industry to fight this foreign competitor. And an older Wired article notes how De Beers wants the government to regulate how faux diamonds are labeled, sold and even displayed (just like Margarine is in several States).
It is “unbelieveable” (sic).