If you think you've found treasure, you must report it to the Coroner within 14 days via the 'Finds Liaison Officer'. It's a criminal offence not to do so.
You can find out what counts as treasure on the Portable Antiquities Scheme site.
You can report your find to the ‘Finds Liaison Officer’ for Essex by emailing lori.rogerson@colchester.gov.uk.
The coroner will contact the relevant body to investigate your find. If your object is confirmed by The British Museum as Potential Treasure, the coroner will hold an inquest to decide whether to declare it “Treasure”.
GOV.UK gives further information on treasure finds.