The influence of thermal and chemical treatments on the physico-chemical properties of the opal-cristobalite rock-flask was studied, as a result of which it was established that the rock under study is a promising sorption material. On the modified flask, sorption of formaldehyde was carried out and optimal modes of modification were established. The sorption characteristics of the modified flask were determined, and optimal regimes for thermal and chemical treatments were established. A technological scheme for modifying the flask for obtaining a sorbent for formaldehyde is proposed.