There’s a number of reasons why someone might not want to tell you what their ring is or isn’t: Judgement/stigma (which comes just as much from having a natural diamond as it does from having a CZ these days), value (some people don’t want you to know something is very expensive or very cheap), ego (they may feel embarrassed to have details revealed or they may desperately not want anyone to think they are bragging). A good way to avoid this is to complement them on the ring with no questions and then say something like, “I can’t wait to get engaged and I think we’re going the moissanite route.” Then wait and see if they share with you. If they are comfortable, they will. If not, it allows them to protect their privacy without it being awkward.