Agate is identical with quartz in composition and physical properties. Composed of silicon dioxide (SiO2), it has hardness of 7, a glassy luster, a conchoidal fracture, and a specific gravity of 2.60. Most agates occur in cavities of eruptive rocks or ancient lavas. These agates have a banded structure, successive layers being approximately parallel to the sides of the cavity. During crystallization, the colored bands are not disturbed. Varieties of agate are characterized by peculiarities in the shape and color of the bands.