It consists of two hills that are joined together by a narrow isthmus. The southwestern part of the island is composed of thick layers of pumice, which are deposited there by two explosions after the formation of Nisyros. The first eruption left on shallow seas over 160 meters of thick layers of pumice, which the company LAVA Mining & Quarrying exploits since 1956. The second explosion leaves a layer of pumice stone 3-4 meters thick in Gyali while it also covers Nisyros with pumice stones that are almost a meter thick.

The northeastern part of the island is composed entirely of thick natural streams of glass (hence the name of the island), like the well-known obsidian and perlite.