"There is a big diamond in the piece of jewelry."
Translation:Korussa on suuri timantti.
Duolingo has made me look this up... Yes, you are largely correct (haha), but they can often be use interchangeably: https://www.kielikello.fi/-/suurella-sydamella-tai-isolla.
Since this is a concrete item, and not huge on an objective scale, I guess a person using "suuri timantti" just wants to sound a little bit more formal or fancy than someone saying "iso timantti".
But yes, way beyond the scope of this course -- either should be accepted as correct.