In 1840, Queen Victoria married her beloved Albert. The engagement ring that he presented to her was a snake with an emerald-set head. The snake was a symbol of eternal love and emerald was her birthstone.
Victoria’s wedding dress was decorated with hand made lace and adorned with a sapphire and diamond brooch, presented to her by Albert, the day before their wedding.
via Lang Antiques.
Now correct me if I am wrong but the Prince Albert Brooch has a very similar styling to the recent Royal Engagement Ring. The choice of a serpentine engagement ring is a trend which has not lasted to this day but was subsequently fashionable in the 19th century.
Sapphires are associated with fidelity and a gift of sapphire suggests trust, honesty and loyalty which is why it is commonly used as a stone for engagement rings. I have heard it said that the blood that runs through the royals is blue so maybe the sapphire is also a symbol of their status and nobility.