Pontnewydd Cave is a remarkable site, providing the earliest evidence of hominid activity in Wales. The 1,284 artefacts are 225,000 years old, and lie within a derived context in the debris flows that form the sedimentary sequence in Pontnewydd Cave. It is almost unique among British Palaeolithic sites in possessing a lithic assemblage made on predominantly silicic igneous rocks. The artefacts were studied in hand specimen and with a petrological microscope, and were classified into petrological groups. The mineral assemblages observed in the thin sections were compared with published descriptions and some examples of Ordovician igneous rocks. Where possible, the provenance of the rock was deduced. The majority of exotics that could be traced back to their original source derived from North Wales, and principally the Snowdonia area, but a smaller number also derived from the English Lake District. These results are consistent with those of previous studies (Bevins 1984). A database of the tool types used at Pontnewydd, their dimensions, and the corresponding rock types from which they are manufactured was compiled. This database was then used to discuss whether some raw materials have been selected over others for artefact manufacture, and if so, whether different suites of raw materials have been used for certain tools. The results indicate that an overall preference has been exhibited for more silicic rocks, and that tools that required less refinement such as handaxes and cores were made on the denser lavas, whilst items that required retouch such as retouched flakes and scrapers were made on the more homogenous flint, chert and fine silicic tuff Through optical microscopic studies of the heavy minerals from many of the layers within Pontnewydd Cave, it was possible to ascertain the approximate geological source of the sediments. Observation of the heavy minerals and the particle size distribution of the sediment also provided some information about the degree of weathering that the sediments were subjected to prior to their emplacement in the cave. This information supported the approximate chronology provided by Embleton and Livingston (1989) and provided new evidence for the source and environment of some of the layers from the New Entrance.