When married parties separate any jewelry of the parties acquired after marriage and before separation is a matrimonial asset and subject to the rules of equalization. Most engagement rings are acquired before marriage and do not go into the calculation of net family properties.
There have always been competing legal theories about ownership of the engagement ring when the wedding is called off. At common law the ring was in the nature of a pledge, to bind the contract of marriage (Jacobs v. Davis,  2 K.B. 532). If the lady broke the engagement off, the ring should be returned. If the gentleman broke the promise to marry, he could not obtain the ring back.
The Marriage Act of Ontario contains the following provisions:
"32. (1) No action shall be brought for a breach of a promise to marry or damages resulting therefrom."
"33. Where one person makes a gift in contemplation of or conditional upon their marriage to each other and the marriage fails to take place or is abandoned, the question of whether or not the failure was caused by or was the fault of the donor shall not be considered in determining the right of the donor to recover the gift."
A previous line of Ontario cases treated the ring as an unconditional gift and as such could not be reclaimed, once given. This appears to be the approach taken by Superior Court Justice Caravan in Kreutzer v. Corbbeil, 2005 CanLll 6395.
The British Columbia courts have imparted far more clarity to this issue than the Ontario courts: unless there is an agreement to the contrary in B.C., the ring is to be returned. This has the effect of restoring the parties to the position they were in prior to the engagement.
Justice Mackinnon appears to take this position in Newell v. Allen, 2012 ONSC 6681 giving summary judgment in favour of the gentleman, ordering that the ring be returned.
Delay in demanding the return of a ring may suggest that the character has changed from an 'engagement ring' to an unconditional gift in which case it need not be returned (Mc Arthur v. Zaduc 2001 CanLll 28143 (ON SC).