Agate
| Agate | |
|---|---|
Polished agate nodule from Malawi, Africa | |
| General | |
| Category | Tectosilicate minerals |
| Group | Quartz group |
| Formula | SiO2 (silicon dioxide) |
| IMA status | Variety of quartz (chalcedony) |
| 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 variously translucent, banded variety of chalcedony. Agate stones are characterized by alternating bands of different colored chalcedony and may also include visible quartz crystals. 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 not true agates. Moreover, not every banded chalcedony is an agate; for example, banded chert forms via different processes and is opaque. Agates primarily form as nodules within volcanic rock, but they can also form in veins or silicified fossils. 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 treated to enhance their color.