This paper aims to evaluate the optical properties of natural rhyolite obsidians that might be useful in the identification of different sources of archaeological samples. Even though colour, degree of alteration, crystaloclast composition and texture are important properties by which sources can be identified, all of these features do not always lead to an unambiguous distinction. In this regard, the refractive index (ND) becomes a helpful tool because it is not only sensitive to the chemical composition of natural glasses but also to their different thermal histories. Obsidians from six localities of Patagonia were analysed. Their refractive index measurements (ND) only range between 1.47 and 1.49, but the ND values of some of these sources are sufficiently constrained to allow discrimination between them.