The most resilient remains of plants in most archaeological sites are the siliceous phytoliths – special cells that are partially or completely silicified during the plant’s life. These cells have characteristic morphologies, and thus the phytoliths can often be used to identify the taxonomic affinities of plants brought to an archaeological site. In order to determine what they were used for, other means of analysis are needed. We present her a method to distinguish burnt from unburnt phytolith assemblages. The method is based on measuring the refractive index (RI) of individual phytoliths. The phytoliths even from a single plant have a range of RI values. Burning a phytolith sample causes a shift to higher RI. Comparing burnt and unburnt samples we demonstrate that it is possible to differentiate between them based on the fraction of phytoliths with RI higher than 1-440. This serves as a basis for a simplified mode of measurement that requires only the use of a petrographic light microscope and a mineral oil of RI 1-440. We apply the simplified method to two Natufian samples from Hayonim cave (Western Galilee, Israel).