Keeper or guard rings first appeared in the late 17th century and comprised a ring or rings worn next to or on either side of a wedding ring to ensure it stayed safely on the finger and could not be lost. The rings were usually silver and gold narrow bands set with rose cut diamonds, though in 1808, La Belle Assemblée, a fashion magazine, announced that multi-gem rings or ‘rainbow hoop rings’ were taking the place of diamond guard rings (Scarisbrick, p 86). The keeper or guard rings looked a little like the modern full eternity diamond ring.
An example of such a ring is held in the Royal Collection Trust. It is a diamond set band given to Queen Charlotte in 1761 by George III. Her Lady in Waiting, Charlotte Papendiek wrote in her diary that part of the King’s ‘particular present’ to his bride was ‘a diamond hoop ring of a size not to stand higher than the wedding ring, to which it was to serve as a guard’, https://www.rct.uk/collection/65429/queen-charlottes-keeper-ring.

The shape and purpose of keeper rings began to change in the 19th century. They became more ornate and thicker, often in a plaited or double curb-chain shape. Rather than being used as guard rings, they were given as tokens of friendship. They became intended to be worn alone on the finger, as some were as wide as 1.5cms.
The Mappin & Webb 1900 Catalogue has a section headed ‘Guard Rings’ containing 15 different rings, including ones like the braided rings as seen in the photo at the top of the post and the one above, as well as snake rings, knot rings and even a buckle ring. Slightly later rings also included substantial rings made up of woven bands engraved with flowers and hearts.

References:
Mappin & Webb 1900 Catalogue
Diana Scarisbrick, ‘Rings: Jewelry of Power. Love and Loyalty’, 2007, Thames & Hudson.
