This well made inlaid pin is set in sterling silver and can also be worn on a chain as a pendant.
The inlay appears to be a speckled, dark-blue enamel and abalone shell (iridescent rainbow colors).
It measures 1 5/8" in diameter. Pinback is a safety clasp.
The hallmark indicates it is sterling silver (eagle standard mark) and made by "HDA" in Taxco.
As you can see the silver areas are very darkened but we usually do not polish vintage silver jewelry unless we are sure it is not an applied patina (sometimes artists applied a darkening agent to the silver and by polishing, this finish would be removed and thus alter the intention of the artist). In this case, it is unclear whether the tarnishing is from age or deliberately applied by the artist.