Agate

Agate forms inside cavities in volcanic rock - primarily basalt. When lava solidifies, gas bubbles create hollow spaces. Over millennia, silica-rich groundwater seeps in, depositing microscopic layers of chalcedony on the cavity walls. Each layer represents a change in the fluid chemistry, producing the characteristic banding.

The process is extraordinarily slow - a typical agate nodule took millions of years to fill, layer by micrometer-thin layer. Variations in iron content, trace elements, and fluid pH create different colors in each band. Some agates never fully fill, leaving a central cavity lined with quartz crystals (a geode).

Special varieties form through unique processes. Moss agate contains filamentous inclusions of chlorite or iron oxide that resemble plant matter - they're mineral growths, not actual moss. Fire agate from Mexico and Arizona contains thin layers of iron oxide (limonite) over chalcedony that produce iridescent color play through thin-film interference.

Agate is defined by its translucent, banded chalcedony composition. The banding distinguishes it from jasper (opaque, no banding) and carnelian (translucent, no banding, orange). At Mohs 7, it's hard with a waxy to vitreous luster and conchoidal fracture.

Hold agate up to a strong light - genuine agate's translucency and banding become visible even in darker specimens. The bands follow the contours of the original cavity, creating concentric patterns. In moss agate, the inclusions are three-dimensional - they appear at different depths within the translucent stone.