Agate
| 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 | Tectosilicate minerals, quartz group, chalcedony variety |
| Formula | SiO2 (silicon dioxide) |
| Crystal system | Trigonal (quartz) or monoclinic (moganite) |
| Identification | |
| Color | Often multicolored; commonly colorless, pale blue to black, red to orange, yellow, white, brown, pink, purple; rarely green |
| Crystal habit | Cryptocrystalline silica |
| Cleavage | None |
| Fracture | Conchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 6.5–7 |
| Luster | Waxy, vitreous when polished |
| Streak | White |
| Diaphaneity | Transparent to opaque (usually translucent) |
| Specific gravity | 2.60–2.64 |
| Density | 2.6 g/cm3 |
| Optical properties | Uniaxial (+) |
| Refractive index | 1.530-1.543 |
| Birefringence | Up to 0.004 |
| Pleochroism | Absent |
| Dispersion | None |
| References | |
Agate (/ˈæɡɪt/ AG-it) is a banded variety of chalcedony. Agate stones are characterized by alternating bands of different colored chalcedony and sometimes include macroscopic quartz. They are common in nature and can be found globally in a large number of different varieties. There are some varieties of chalcedony without bands that are commonly called agate (moss agate, fire agate, etc.); however, these are more properly classified solely as varieties of chalcedony. Agates are primarily formed as nodules within volcanic rock, but they can also form in veins or in sedimentary rock. Agate has been popular as a gemstone in jewelry for thousands of years, and today it is also popular as a collector's stone. Some duller agates sold commercially are artificially dyed to enhance their color.