Thanks to Dee for this. I know she’s on her way to Atlanta for the RWA National Conference, so I thought I’d post what she’s helped me find on Victorian engagement rings. Her information was also a reminder to increase my Victorian book collection. I’ll shortly be buying Hands and Hearts: History of Courtship in America.
Queen Victoria also wore an engagement ring, a serpent symbolizing good luck.  And, of course, what the queen wore, society copied. As with many things from the Victorian Era, our traditions date from what they did.
Rings were often given as a sign of a marriage promise, though often only the wealthy could afford to do so. It wasn’t until 860 AD that the Catholic Church mandated all marriages were to be symbolized by a ring. “In 860 Pope Nicolas I decreed that a ring was a requirement to signify betrothal or engagement and it was also stipulated that it should be a gold ring…” 
But that was a wedding ring, not an engagement ring. The first recorded occurrence of a woman receiving a diamond engagement ring was in 1477. The Archduke Maximilian of Hamburg gave it to his betrothed, Mary of Burgundy.  Unfortunately, I wasn’t able to find anything on the ring itself.
“By 1890 engagement had become a distinctive stage in the transition to marriage—a stage with its own rites of initiation (the announcement), its own ceremonial object (the engagement ring) and its own rules of conduct” 
A diamond ring symbolized innocence; sapphires, immortal life; rubies, affection; emeralds, success in love. No pearls or opals – they were considered harbingers of bad luck. 
By the mid 1870’s colorless stones were more fashionable. By the 1880’s, diamonds were considered far better taste than the brightly colored stones of the 1860’s. And by the 1890’s, colored stones were completely out of fashion. In 1857 diamond mines were discovered in Brazil, and in 1867 they were discovered in South Africa. 
3. Hands and Hearts: History of Courtship in America by Ellen K. Rothman
4. Illustrated Encyclopedia of Victoriana: A Comprehensive Guide to the Designs, Customs, and Inventions of the Victorian Era by Nancy Ruhling and John Crosby Freeman
5. Victorian Jewelry by Margaret Flower