The wedding ring on Saul's finger was a simple band of gold, with a tiny diamond inset into it. Sam had saved up many months before going into Scurrick's, on the Street of Cunning Artficers. Though, to be fair, it was easier now that she wasn't drinking her wages.
She had them carve inside only the word "Forever." She wasn't prone to sentiment or speaking her feelings freely, but Saul knew what was meant by it when she placed the ring on his finger.
—-
There was the wedding ring that Sam wore every day on her finger. It was a plain gold band, a practical ring for a hard working Watchman, though she got into the habit of twisting it round her finger for awhile.[1]
And then there was an engagement ring, worn on a chain under her armor and shirt, close to her heart[2], gold with a small ruby. For most, it would be a trifle, a small thing.
But it is very precious to Vimes, so she wears it safe and close to her heart, her actions telling Saul all he needs to know.
[1] Sam never having worn jewelry until she was married. Saul had to laugh sometimes; his mother's jewel box contained pieces the ladies of Society would kill to have and Sam didn't even have her ears pierced.
[2] well, close to her heart-ish.