What is the law in Nevada about getting an engagement ring back from an ex who broke off the engagement?

Your explanation is a bit vague, and Nevada does not have a published Opinion from the Nevada Supreme Court explicitly on point.  However, I have researched this pretty closely (and litigated it a couple of times) and Nevada almost certainly will follow the "general rule" -- an engagement ring is a conditional gift, not effective until the condition (actual marriage ceremony) is completed.  Until then, it belongs to the giver; after that, it belongs to the recipient. If you need further assistance, you should consult an attorney.