This is a 22k gold with full distinct UK hallmarks for 1874 and a gemologist has confirmed to be natural amethyst. Let's do a full break down!
First, amethyst info:
Amethyst is a readily available gemstone and can be found in many different parts of the world. It is formed with intense colors on the outside and at the top. The most valuable is a deep purple color with the least amount of color zoning.
Image taken from Gemsociety, amethyst on matrix
Around and before the 17th century, amethyst was reserved for royalty and the elite in society. This was due to royal purple dye being extremely rare to find. Up until the 19th century, Russia was the main source of amethyst. Presumably, with the hallmarks for this ring is where the Brazil mining has already started to become accessible. It is an intense purple color and I see equal color penetration throughout both gems.
Engagement history
Napoleon’s engagement ring to Josephine sparked a cult like favoritism to the toi et moi design. It’s usually designed with two stones adjacent to each other to symbolize “you and me”. You can see it in modern engagement rings today.
Image taken from Forbes. Josephine's engagement ring sold for 730k Euros:
Queen Victoria’s romance with Prince Albert spurned a lot of fashionable styles and trends during her reign. Prince Albert presented her with an engagement ring with an emerald to represent her birthstone. This catalyzed others to have engagement rings as birthstones. The history of birthstones go far back! There’s record of it being utilized as far as the 16th century and in 1912, was when the American National Association of Jewelers standardized the birthstones that we know today. There’s not a lot of accessible history on birthstone. But Tiffany and Co published a pamphlet in 1870 with a poem about birthstones. In this poem, amethyst was marked as a birthstone in February.
Image taken from NAJ, Queen Victoria's engagement ring
Conclusion
So one can deduct from two very critical and fashionable engagement ring trends: both from Josephine and Queen Victoria that this antique ring is in fact, a Victorian engagement ring. In addition, 22k gold is a very popular gold purity in the UK wedding stack. A luxurious esteemed ring to bring into one's collection!