Tool-stone selection in the African Middle stone age at Sibhudu cave

RESEARCH ARTICLE Tool-stone selection in the African Middle stone age at Sibhudu cave Patrick Schmidt 1,2,3 *, Klaus G. Nickel 4,5 1 Petrology and Mineral Resources, Department of Geosciences, Eberhard Karls University of Tübingen, Germany, 2 Archaeometry, Department of Prehistory, Early History and Medieval Archaeology, Eberhard Karls University of Tübingen, Germany, 3 Department of Archaeology, University of Cape Town, Rondebosch, South Africa, 4 Applied Mineralogy, Department of Geosciences, Eberhard Karls University of Tübingen, Germany, 5 Eberhard Karls University of Tübingen, Competence Center Archaeometry – Baden Wuerttemberg, Tübingen, Germany * Abstract OPEN ACCESS Citation: Schmidt P, Nickel KG (2026) Toolstone selection in the African Middle stone age at Sibhudu cave. PLoS One 21(6): e0350817. https://doi.org/10.1371/journal.pone.0350817 Editor: Marco Peresani, Universita degli Studi di Ferrara, ITALY Received: January 9, 2026 Accepted: May 19, 2026 Published: June 4, 2026 Copyright: © 2026 Schmidt, Nickel. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data availability statement: All data produced for this study are available in the paper and the supplementary material. Funding: PS received funding from the Deutsche Forschungsgemeinschaft (DFG) (grant number 527050193, SCHM 3275/3-1). Competing interests: The authors have declared that no competing interests exist. The raw materials people used for making stone tools may contain information about their territory, exchange routs or the selection criteria they employed during provisioning. In this study, we measure the mechanical properties of different tool-stones used by foragers living during the Middle Stone Age at Sibhudu Cave in South Africa. The site yielded a long and continuous sequence that saw transitions between different raw materials and tool forms. We evaluate the quality of these different stones for tool making and use, attempting to find correlations between selected raw materials and the tools made from them. We find that the raw materials used at Sibhudu have substantially different qualities, some being easy to flake but weak upon use, some being tough during stone knapping and resistant during use. Comparing these data with the appearance and disappearance of tool types throughout the Sibhudu sequence, we note that tool-stones requiring lower flaking forces were more often retouched than those requiring great forces. Elongated products, blades, were mostly made from materials with better fracture predictability, suggesting an understanding of the basic requirements for standardising the tool knapping process. Use-related qualities, such as resistance to dulling, appear to have been of lesser importance at Sibhudu. Our results suggest that the site’s occupants had a good understanding of the qualities of rocks for specific knapping processes. Introduction The African Middle Stone Age (MSA) has seen major innovations, regional patterns in the stone toolkit [1] and new human forms. In southern Africa, the second half of the MSA is associated with Homo sapiens [2]. During this period, discrete techno-cultural entities build a diverse sequence with sub-stages identifiable across the subcontinent [3]. These sub-stages were also associated with major transitions in the raw materials PLOS One | https://doi.org/10.1371/journal.pone.0350817 June 4, 2026 1 / 11 used for tool making [4]. It has been argued that raw material procurement patterns reflect the territories of foragers [5,6]. Others proposed that raw material choices were driven by the need for certain tool-stones and the knowledge of the mechanical properties that are useful for specific tool types [7]. Measuring the mechanical properties of raw materials may help understand which of these two scenarios was more likely to be true [8,9]. However, the comparison of different materials can only be conducted at archaeological contexts, which yielded long and continuous sequences and where different raw material types occur throughout the sequences. In South Africa, such archaeological sequences are rare. One example is Diepkloof Rock Shelter on the western seaboard [10]. There, it has recently been argued that foragers selected specific tools-stones to manufacture stone tools [8]. So far, Diepkloof provides the only available data set on mechanical properties of raw materials in the MSA. Another site with a long and continuous sequence is Sibhudu Cave (henceforth only Sibhudu) on South Africa’s eastern seaboard [11]. The site yielded a sequence from the pre-Stillbay (SB) to the post-Howiesons Poort (HP), where transitions of different raw materials can be tracked through time [12]. In this study, we analyse the mechanical properties of tool-stones found in the Sibhudu sequences. For this, we employ an approach based on standard measurements commonly employed in engineering [for an overview see for ex.: 13]. The measurement of standard mechanical properties, such as strength, fracture toughness or hardness, has been widely applied to archaeological stone raw materials from different parts of the world [e.g., 14, 15–17]. Standard quantities provide comparable data across different contexts and researchers. However, a clear link to what archaeologists call knapping quality was missing until recently. Nickel and Schmidt [9] proposed a theoretical framework allowing to combine the standard measurements fracture toughness, strength, hardness and stiffness to make predictions on the force needed to induce flaking. This allows comparing knapping force requirement in different tool-stones, establishing an objective ranking of different materials. The absolute values of such force predictions must be regarded as approximations of the force requirements in real world knapping because they are based on a fracture initiation mechanism (mode I fracture opening by indentation) that is different from that in stone knapping (i.e., Hertzian fracture). However, values obtained on different samples are comparable and the absolute values are overall not too far off from the force requirements in real stone knapping (compare for example the knapping force values in [18] and [8]). Here we apply the refined model proposed by Schmidt et al. [8] to make such force predictions. Another measurable aspect of knapping quality has been proposed to be the predictability of the raw materials’ fracture behaviour [19,20]. We measure fracture predictability with an approach based on Weibull’s [21] fracture theory. The advantage of such Weibull statistics over the more traditionally used ‘impurity encounter rate’ [19] is that the approach provides comparable values derived from actual tests of the failure behaviour of rocks. Both these measures, fracture force and predictabili (...truncated)