Canadian law regarding importation of and duty due on articles acquired abroad by persons normally resident in Canada is described on this Canadian government webpage. If the ring was originally acquired by your fiancé in Canada, and your fiancé can so demonstrate upon your return to Canada, no duty will be due.
If the ring was obtained by your fiancé in the US, it is not possible to answer your question without knowing the duration of your stay outside Canada.
Different exempt amounts (and the method for their computation) apply depending upon the duration of your trip outside Canada. The above-cited page presents four duration categories: less than 24 hours outside Canada, 24 hours to 48 hours outside Canada, 48 hours to 7 days outside Canada, and more than 7 days outside Canada. Each duration category has different terms for computation of duty.
No matter the duration, you must declare the ring (and anything else you acquired while out of Canada) to the Canadian Border Services officer when you reenter Canada.
In Comments, @mlc has pointed out that the cited Canada customs page also contains this:
Under the North American Free Trade Agreement (NAFTA), no duty is payable on goods imported for personal use if the good is marked as made in Canada, the United States of America (USA) or Mexico, or if there is no marking or labelling indicating that it was made somewhere other than in Canada, the USA, or Mexico.
Thus, the original answer is incorrect if the goods imported for personal use are marked as made in Canada, the USA, or Mexico.
However, it is not known how Canada Border Services will choose regarding an item, like a ring, which is likely too small itself to contain a "marked as made" label or engraved text.