Hyalite

Hyalite, also known as glass opal, is a variety of opal characterized by its clear, colorless to slightly colored texture and characteristic glassy luster. This mineral is composed of silica (SiO₂) and contains water, which gives it its unique properties. Hyalite is formed by hydrothermal processes near the Earth's surface, often in the cavities of igneous rocks such as basalt. In the Czech Republic, there is an important deposit of hyalite in the vicinity of Valec, where it occurs as grape-like or stalactite-like aggregates.