The knowledge of the structure/property relationship in polycrystalline materials is the basis for successful application of Grain Boundary Engineering. We demonstrate this relationship in the reverse way: from the selective corrosion attack observed in unique sample – loops from excavated necklace dated to the 10th century and manufactured from a Ag–1%Cu alloy – we can deduce the method of manufacturing the objects. Individual grain boundaries in this object were identified by electron back-scattering diffraction. Crystallographic maps of the grain boundaries are confronted with the level of the long-termed selective corrosion attack in the soil electrolyte under conditions of decomposing human body. It is shown that general grain boundaries, which are highly segregated by copper, are preferably attacked by corrosion. The segregated layers represent a less-noble material comparing to the surrounding bulk in this environment. In contrast, the twin and other special grain boundaries are significantly more resistant against this attack.