If it is any help the very pious Holy Roman Emporer Charles V (i think it was him, or maybe the HRE before him…) was the first guy to give a diamond ring for an engagement. So a Christian added to this tradition even though it was originally pagen.
Christianity adopted alot of pagan symbols to help make Christianity make more sense to them. Thats why easter is in Spring a time of new life. That is why we celebrate the dead on All Saints Day and All Souls day and throughout november as nature is dieing for the winter. We celebrate Christmas in the winter just after the darkest day of the year because it marks hope returning to the world.
We take tones of traditions from pagans but the traditions are not bad as long as we use them to glorify God in some way. Marriage rings symbolize the marrage and as long as the couple is always faithful and live their lives together for Chirst then their marriage is glorifying to God and so the symbol of their marriage must be glorifying to God by proxy.
Hope that makes sense and helps.