The study of the gold earrings from the Recouso Treasure (NW Iberia) led us to identify the use of non-metallic fills in the manufacturing process. This article offers an initial characterization of these core materials and discusses their possible purpose. Based on the identification of diatom skeletons in the fill of one earring and the Py-GC-MS analysis of a sample from another, we suggest that sediment from a stagnant open body of water was used as core material. This sediment was subjected to thermal modification. Although the use of non-metallic cores in goldworking processes has been documented in other areas of Atlantic Europe, such as the British Isles, studies of their nature and composition are few. The identification and radiocarbon dating of charcoal samples from an Ag–Au–Cu melting mass from the same hoard are also included in this article. The charcoal fragments were identified as oak (Quercus sp. deciduous) and dated to the second to first centuries BC.