sedimentary
Banded Agate
Silicon dioxide (SiO2) - variety of Chalcedony
Hardness: 6.5-7 on Mohs scale; Color: Multi-colored bands (white, brown, red, gray); Luster: Waxy to vitreous; Crystal Structure: Trigonal (cryptocrystalline); Cleavage: None; SG: 2.58-2.64
- Hardness
- 6
- Color
- Multi-colored bands (white, brown, red, gray)
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Multi-colored bands (white, brown, red, gray); Luster: Waxy to vitreous; Crystal Structure: Trigonal (cryptocrystalline); Cleavage: None; SG: 2.58-2.64
Formation & geological history
Formed primarily in volcanic and metamorphic rocks as a result of deposition of silica from groundwater in cavities (vesicles). It often occurs in ancient basalt or andesite formations.
Uses & applications
Used extensively in jewelry (beads, cabochons), ornate carvings, bookends, and historically for precision mechanical components like mortar and pestles due to its hardness.
Geological facts
Agate is named after the Achates River (now the Dirillo River) in Sicily, where it was first discovered by the Greek philosopher Theophrastus. Its banded appearance is caused by alternating layers of chalcedony and quartz.
Field identification & locations
Identify by looking for concentric or parallel curved bands. In the field, look for nodules with a waxy surface texture. Commonly found in riverbeds and on beaches in volcanic regions like Brazil, India, and the Lake Superior region of the USA.
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