During the Iron Age the west of Armorica was set apart from "Celtic Europe" by two main features : the carving of granite stele and the digging or underground passages near settlements. This practice lasted some four hundred years (from the 5th to the 2nd centuries).
The underground system at Treglonou in associated with a true oppidum covering a surface or about 20 hectares (almost 50 acres); it consists of a vertical shaft giving access to two small low-cellinged chambers ; linked by a narrow passage. The underground structure ends in a narrow sloping alley emerging in the outer ditch of the defence system. It was in this alley that the beads of a gold necklace were found.
Each bead is composed of two embossed gold shells, decorated with both chased and repoussé motifs. The length of the beads varies from 1,6 to 2,5 cm and their weight from 2 to 4,4g : the total weight of the necklace is 37g. The assembly method used for the shells, studied by C. Eluere and A.R. Duval is very delicate ; it calls on the copper diffusion bonding process.
These beads constitute a unique ornament, with no known equivalent in Europe. Certain comparisons may be made with Late Bronze Age pinheads found in Switzerland, Hallstatt period jewellery from Alsace, southern Germany and Switzerland, and certain beads from the Late Bronze Age in Ireland and Great Britain.
The Treglonou beads could have a local origin and have been made between the 9th and 5th centuries BC. They were deposited, hidden, or lost, at the time the site was scaled up by its Iron Age occupants around 300-400 BC.