This is an obvious intaglio made out of Hematite and is very low artistic quality.
In some countries hematite is called "blood stone" because the hematite dust is a red color. Heliotrope is another blood stone that is green chalcedony with red inclusions of iron oxide or red jasper.
This locket is made in England specifically to wear it with the pocket watch, so it's actually a pocket watch fob. These fobs were made from gold and also many were gold filled. That would be determined by acid test.
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Hard-Stone Cameo or Intaglio Locket? by: Yvonne
Good afternoon!
Tuck, it does indeed look interesting and old. I am thinking it might be an intaglio, is the subject raised above the surface of the flat stone like a cameo relief or not? Intaglio is the name for engraving with negative image. Retrospectively, in days of old, intaglios were used for seals, like a signature today.
Tuck, do you know if this item was originally made as a locket or has it been adapted? The material sounds interesting, of a black color under light, dark crimson red...
... bloodstone was used as a stone for seals, but it is green and has red spots resembling blood. Possibly, the stone in heirloom family locket is a hard-stone like onyx or jasper, not sure, definitely not shell that is used for cameo making though.
As the piece is not set with gems, it could be appraised on-line by experts in this area, however, the marks on the reverse would need to be examined for attestation. Close-up images in focus would work to your benefit. The on-line appraisal form can be found here.