Look at Moissanite. Most people can't tell its not a diamond, and its as hard as one. Found one on amazon in a 5 second search that is sterling silver and 2 carats for $160. You certainly have options. Be honest with her about the materials so she can care for it properly!

EDIT: Actually it was platinum plated sterling silver on that ring.

https://www.amazon.com/Moissanite-Engagement-Brilliant-TransGems-Platinum-plated-silver/dp/B071S7W555/ref=sr_1_4?s=apparel&ie=UTF8&qid=1509556944&sr=1-4&nodeID=7141123011&psd=1&keywords=moissanite+engagement+ring&dpID=417WO-UVSpL&preST=_SY300_QL70_&dpSrc=srch