If you create a "diamond" out of plastic, then you’ve created a factitious diamond, meaning that it’s a phony.
Factitious, pronounced "fac-TISH-us," means "fake," like a factitious compliment you give the person who cooked you an awful meal — you don't mean it, but you say it anyway, just to be nice. As he or she happily walks away, another friend might whisper, "Were you being facetious about the dinner being good?" Facetious, pronounced "fuh-SEE-shuhs," means "trying to be funny." Don't confuse factitious with facetious — or fictitious, which means "made up."