Pair of Turquoise Locket rings

Curious pair of small 18ct turquoise locket rings c.1870.

Makers marks on one of the pair, four letters in diamond shaped stamp.

Both of the rings are set with a single cabochon turquoise with a tiny locket behind, presumably for a lock of hair from the other.

Perhaps for sisters or friends or lovers, the original story is now lost.

Turquoise in c19th sentimental jewellery signified forget-me-not and spoke of true love, affection, and enduring fidelity.

Size 6.25 and 6.5. 
2 grams each.