This article documents the significant horizontal (across the landscape) and vertical (across the stratigraphy) chemical variability of volcanic clays from Vanuatu, South Pacific. Data illustrate why the chemical composition of the clay matrix in pottery should be used very cautiously in characterization or provenance studies. The variability of natural clays on Efate, Erromango, and Malekula is so significant that the data set disproves the assumption that two pottery samples with clay matrices showing similar chemical composition necessarily originate from the same location, the same bedrock, the same region, or even from the same island. This study is also a reminder that the outcomes of chemical characterizations and provenance studies of pottery are directly dependent on the scale at which the investigation is undertaken. In light of the data, it is also clear that such studies should not be undertaken in other similar insular environments affected by regular volcanic activity along the Circumâ€Pacific Belt without assessing the natural variability of the raw material. Without an adequate sampling of natural clay representative of the vertical and geographic variability, the results from the chemical analysis of clay matrices risk of leading to incorrect associations between pots and procurement areas.