For the ultimate expression of love in an organic, carbon-neutral way: Growing Jewellery by Icelandic designer Hafsteinn Juliusson.
The rings represent
"... a redefinition of modern values. It is a clash of jewelry and gardening; couture and organism. The collection is designed for people in metropolitan cities and is an experiment in drawing nature toward man, as nature being the presupposition of life."
Tiny Icelandic moss, which requires watering, is planted in the ring, and can they can last up to six months.*
*The ideal length of an engagement -- longer than that, and the flowers/centerpieces/ menu/ fittings/meddling/ etc. will drive you mad.**
** Unless you like that sort of thing... that's cool.
**aI'm still recovering from the wedding year... It might have been nice to have a ring with an expiration date.