*Note: Not all these couples are engaged or married in the books. I'm looking down the line (and sometimes just wishfully dreaming) for some of them.
Cath and Levi from Fangirl by Rainbow Rowell
Elisa and Hector from The Girl of Fire and Thorns series by Rae Carson
Something about this ring reminds me of a Godstone, which is what made me think of Elisa.
Harry and Ginny from the Harry Potter series by J.K. Rowling
Pretty sure Ginny wouldn't want anything too fussy. Can't have any big rocks getting in the way of her Quidditch playing!
Jay Gatsby and Daisy from The Great Gatsby by F. Scott Fitzgerald
On the other hand, the gaudier and the more extravagant, the better for Jay and Daisy!
Alina and Mal from the Grisha series by Leigh Bardugo
Time will tell how this series ends, but if Alina picks Mal, I'm thinking something really simple and sun-like would be perfect.
Alina and Sturmhond from the Grisha series by Leigh Bardugo
If she picks Sturmhond, though, something very different will be glittering from her sun summoning, princess hand.
Alina and the Darkling from the Grisha series by Leigh Bardugo
And we can't ever rule out The Darkling. As terrifying and powerful as this couple would be, they would also be EXTREMELY AWESOME. Like this ring.
America and Prince Maxon from The Selection series by Kiera Cass
This ring is feminine and royal, and for some reason kind of reminds me of the cover image.
Kestrel and Arin from The Winner's Curse by Marie Rutkoski
I like the combination of the two different stones, and think it would well represent the diversity between Kestrel and Arin.
Hamlet and Ophelia in Hamlet by William Shakespeare
Let's pretend things went REEEEEAAAALY differently from how Shakespeare wrote them. In that case, this could suit them just fine.
Zoe and Darian from Troubled Waters by Sharon Shinn
This ring would suite a Coru girl perfectly, though Zoe would probably think it was too showy and "forget" to wear it half the time.
Bitterblue and Saf in Bitterblue by Kristen Cashore
This ring just screams fantasy to me. And I like the touch of blue.
Taylor and Jonah in Jellicoe Road by Melina Marchetta