This dataset contains Volume and density measurements for obsidian using helium pycnometry. The data was collected and analysed at the University of Liverpool for cored rhyolitic obsidian cylinders from Hrafntinnuhryggur, Iceland. The geographical location of the samples is here inconsequential for the dataset; the specific obsidian was chosen for its physical properties alone. The sample mass (using a scale) and geometry (using a caliper) of 7 representative samples of rhyolitic obsidian was measured, and the samples were analysed using a helium pycnometer, which measures volume, and correspondingly density when the sample mass is provided. The density of the rhyolitic obsidian was needed for the experiments in the study Seropian et al (2022).