agate

Agate is a form of chalcedony, a cryptocrystalline variety of quartz (SiO2). It is typically translucent to opaque and prized for its smooth texture and characteristic banding, which comes from successive layers of microcrystalline quartz and impurities.

Agate forms mainly in volcanic rocks when silica-rich fluids deposit thin layers inside cavities and fractures. Over time these layers build up into concentric or irregular bands. Colors and patterns are influenced by trace elements and environmental conditions during growth.

Common varieties include fortification agate, with broad, connected bands; moss or dendritic agate, featuring tree-like mineral inclusions; and fire agate, which shows iridescent color caused by light interference within inclusions. Many agates are also dyed to enhance color.

Geographic distribution is widespread. Important sources include Brazil and Uruguay, as well as Mexico and several locations in the United States, such as the Lake Superior region and parts of Oregon and Montana.

Uses and care: Agate is widely used in jewelry and decorative objects. It ranks about 7 on the Mohs scale and accepts a high polish. Natural agate colors occur, but many stones are dyed. Clean with mild soap and water; avoid harsh chemicals or ultrasonic cleaners, especially for dyed specimens.