agat

Agat, known in English as agate, is a form of chalcedony, a microcrystalline variety of quartz. It is typically translucent to opaque and occurs as nodules or fillings in vesicles of volcanic rocks. The distinguishing feature is its banded appearance, produced by the accumulation of silica in successive layers over time.

Chemically, agat is mainly silicon dioxide (SiO2). Its color and patterning arise from trace impurities such as iron, manganese, or cobalt and from optical effects within its microcrystalline structure. Banding can be concentric, parallel, or more complex, as seen in moss agate with dendritic inclusions and in other patterned varieties.

Common varieties include banded agate, fire agate with iridescent flashes, blue lace agate, and Botswana agate. Nodular forms known as thundereggs are often cut as cabochons or geode-like specimens. The overall color range is broad, including whites, grays, browns, reds, blues, and greens.

Occurrences and sources: agat is widely distributed worldwide. Major sources include Brazil, Uruguay, Mexico, the United States (notably Oregon, Montana, and Arizona), Botswana, Madagascar, India, and Australia. It is widely used in jewelry, beads, carvings, and decorative objects.

Care and treatment: agat ranks around 6.5–7 on the Mohs scale and is relatively durable. It is usually cleaned with warm, soapy water. Some agat is dyed or heat-treated to enhance color; treated material should be disclosed by sellers. Etymology traces the name to classical sources linked to the river Achates in Sicily, reflecting ancient trade in this mineral.