agates

Agates are a diverse form of chalcedony, a microcrystalline variety of silica. They are characterized by layered, banded patterns that arise as silica-rich solutions precipitate in tiny voids and cavities within volcanic rocks. The resulting bands can be concentric, irregular, or fortification-like, giving each specimen a distinct appearance.

Formation and structure occur when groundwater or hydrothermal fluids deposit successive layers of silica in open spaces such as gas bubbles and fractures in lava. Over time, the repeated deposition creates the characteristic translucent to opaque bands. Large agate-filled nodules, or thundereggs, are a notable example of this process.

Key varieties include fortification agate, which shows intricate, maze-like banding; moss or mossy agate, with dendritic inclusions that resemble plant forms; fire agate, known for a striking iridescent play of color produced by internal microstructures and impurities; blue lace agate and crazy lace agate, which display distinct color patterns. Laguna agate from Chihuahua, Mexico, is among the most prized forms for its sharp banding and vivid coloration.

Agates occur worldwide, with notable deposits in Brazil, Uruguay, Mexico, the United States (notably Oregon, Montana, and California), India, Madagascar, and Botswana. They are commonly cut into slabs, cabochons, or beads for jewelry and decorative objects. On the Mohs scale, agate hardness is about 6.5 to 7, contributing to their durability in wear. They are often dyed or heat-treated to enhance color; some rough material is resin-stabilized for stability.

The name derives from Greek achates, after the Achates River in Sicily, reflecting the long history of human use and collection of these stones.