What makes a mineral glassy?

A mineral is considered glassy when it has an amorphous structure rather than a crystalline one, resulting in a smooth, shiny surface that reflects light. This non-crystalline arrangement allows for a lack of defined cleavage planes, contributing to a glass-like appearance. The cooling process of molten rock can lead to the formation of glassy minerals, as rapid cooling prevents the orderly arrangement of atoms typically seen in crystalline minerals. Common examples of glassy minerals include obsidian and some volcanic glasses.