Mineral

Banded Agate (Brazilian Agate Slice)

Chalcedony (Silicon Dioxide / SiO2)

Hardness: 6.5–7 (Mohs scale); Color: Variegated levels of grey, blue, and brown; Luster: Waxy to vitreous when polished; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.60–2.64.

Hardness: 6
Color: Variegated levels of grey, blue, and brown
Luster: Waxy to vitreous when polished

Identified March 24, 2026More mineral →

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Physical properties

Formation & geological history

Formed in volcanic lava cavities (vesicles) where silica-rich groundwater deposited successive layers of microcrystalline quartz over thousands of years. Known to be common in deposits dating back to the Triassic and Cretaceous periods.

Uses & applications

Used primarily in lapidary arts for jewelry, bookends, and decorative coasters. Industrially, it is used to make chemical-resistant mortars and pestles because of its hardness and toughness.

Geological facts

Agate belongs to the quartz family but is distinguished by its 'cryptocrystalline' structure, meaning the crystals are too small to be seen without a microscope. The banding patterns are often referred to as 'water-line agates' if straight, or 'fortification agates' if they mimic the angles of a fortress.

Field identification & locations

Identify in the field by looking for translucent nodules with a waxy exterior or concentric banding patterns when broken. Common locations include Brazil (Rio Grande do Sul), Uruguay, Mexico, and the Lake Superior region in the USA.

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