What is "gyali"

Gyali ( " glass", also spelled Giali or Yali, pronounced ) is a volcanic Greek island in the Dodecanese, located halfway between the south coast of Kos ( Kardamaina) and Nisyros. It consists of rhyolitic obsidian lava domes and pumice deposits (which are mined in huge quantities). The island has two distinct segments, with the northeastern part almost entirely made of obsidian and the southwestern part of pumice. These are connected by a narrow isthmus and beach made of modern reef sediments.