Agate
Silicate (chalcedony) · SiO₂
Agate is a banded form of chalcedony (microcrystalline quartz), famous for its concentric colour bands and popular with rock tumblers and cutters.
What is agate?
Agate is a variety of chalcedony, itself an extremely fine-grained form of quartz. It forms in cavities in volcanic rock where silica-rich water deposits layer after layer, producing the concentric bands agate is known for. It is tough, takes a high polish and comes in endless colour patterns, making it the classic tumbling and cabochon stone.
Properties
- Chemical formula
- SiO₂
- Category
- Silicate (chalcedony)
- Hardness (Mohs)
- 6.5–7
- Crystal system
- Trigonal (microcrystalline)
- Lustre
- Waxy to vitreous
- Streak
- White
- Colour
- Banded; every colour
- Cleavage / fracture
- None; conchoidal fracture
How to identify agate
- →Concentric or wavy colour banding, often in a rounded nodule.
- →Hardness near 7, scratches glass, waxy to glassy lustre.
- →Translucent on thin edges when held to light.
- →No visible individual crystals (microcrystalline).
Where agate is found
Agates are found worldwide in volcanic regions and as beach and river pebbles. Famous localities include Brazil, Mexico, Botswana, Scotland and the Lake Superior region of the USA.
Agate finds on minShelf
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