Jake Webster found the post medieval silver seal matrix nine inches under the soil in a field near Llawhaden on January 20, 2022.
A report compiled by Sian Iles from the National Museum of Wales said that the object was a small complete silver seal matrix with an oval face and octagonal open work.
The seal was engraved with a shield and a coat of arms with six stars on it. It measures 23.4 mm with a diameter of 15.6mm by 13.3mm. The handle of the seal measures 10.7mm and it weighs 5.6 grammes.
The report said that the object was a heraldic seal matrix made of an alloy of more than 10 per cent silver.
Its composition and design is consistent with other finds of this sort from the 17th century.
The treasure inquest said that attempts to identify the coat of arms have been unsuccessful, but it was close in appearance to a house in Cambridge.
Deputy HM Coroner for Pembrokeshire, Gareth Lewis, declared the item treasure as it was more than 300 years old and made up of at least 10 per cent silver.
He added that Tenby Museum was interested in acquiring the item.