Agate
Agate (/ˈæɡɪt/ AG-it) is the banded variety of chalcedony, which comes in a wide variety of colors. Agates are primarily formed within volcanic and metamorphic rocks. The ornamental use of agate was common in Ancient Greece, in assorted jewelry and in the seal stones of Greek warriors, while bead necklaces with pierced and polished agate date back to the 3rd millennium BCE in the Indus Valley civilisation.
| Agate | |
|---|---|
19.6 kg (43 lb) specimen of "Crazy Lace" agate from Chihuahua, Mexico next to a tennis ball; 38.2 cm (15.0 in) wide | |
| General | |
| Category | Chalcedony variety |
| Formula (repeating unit) | SiO2 (silicon dioxide) |
| IMA symbol | Aga |
| Crystal system | Trigonal or monoclinic |
| Identification | |
| Color | banded |
| Crystal habit | Cryptocrystalline silica |
| Cleavage | None |
| Fracture | Conchoidal, with very sharp edges |
| Mohs scale hardness | 6.5–7 |
| Luster | Waxy |
| Streak | White |
| Diaphaneity | Translucent |
| Specific gravity | 2.58–2.64 |
| Refractive index | 1.530–1.540 |
| Birefringence | up to +0.004 (B-G) |
| Pleochroism | Absent |
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