I suspect that as long as you declare it when you re-enter Canada you will have no issues (other than a potential liability to pay duty and taxes). If you don't declare it and get "caught" I suspect you'll have lots of issues. You can always call the CBSA for information - their toll-free number is 1-800-461-9999.

As per the CBSA website:

When you return to Canada, you have to declare all of the goods you acquired while outside Canada, such as purchases, gifts, prizes or awards that you are bringing with you or are having shipped to you. Include goods that are still in your possession that you bought at a Canadian or foreign duty-free shop. As well, make sure you declare any repairs or alterations you made to your vehicle, vessel or aircraft while you were out of the country.

If you aren't sure if an article is admissible or should be declared, always declare it first and then ask the border services officer. Remember that officers are there to help you and will work out your personal exemption and any duty and taxes you owe in the way that benefits you most.

Edit: You didn't say if you would be gifting the ring to your SO when you come back to Canada or if your SO is actually in the US and will be receiving it at a later date (in the US). This could have a bearing on your case.