glassy texture

Volcanic glass is formed when lava or magma cools so rapidly that it doesn't crystallize, resulting in a glassy texture. This typically happens when molten material with a high viscosity, close to granite's chemical composition, is suddenly chilled, often as it erupts from a volcano.1

Obsidian is a common type of volcanic glass, rich in silica and formed by the rapid cooling of viscous lava. While volcanic glass is unstable and tends to crystallize over time (a process called devitrification), geologically ancient glassy rocks are rare because most have since transformed into a stony appearance.21

Igneous rocks can be classified by their crystallinity, ranging from entirely crystalline (holocrystalline) to entirely glassy (holohyaline or hyaline). Rapid cooling, characteristic of volcanic environments, leads to aphanitic (fine-grained) or glassy textures.3