Onyx
| Onyx | |
|---|---|
Onyx with concentric banding | |
| General | |
| Category | Tectosilicate minerals, quartz group, chalcedony variety, agate variety |
| Formula | SiO2 (silicon dioxide) |
| Crystal system | Trigonal (quartz), Monoclinic (moganite) |
| Identification | |
| Formula mass | 60.08 g/mol |
| Color | Black and white; red to brown with black or white (sardonyx) |
| Cleavage | None |
| Fracture | Uneven, conchoidal |
| Mohs scale hardness | 6.5–7 |
| Luster | Vitreous, silky |
| Streak | White |
| Diaphaneity | Translucent |
| Specific gravity | 2.55–2.70 |
| Optical properties | Uniaxial/+ |
| Refractive index | 1.530–1.543 |
| References | |
Onyx is a typically black-and-white banded variety of agate, a silicate mineral. The bands can also be monochromatic with alternating light and dark bands. Sardonyx is a variety with red to brown bands alternated with black or white bands. The name "onyx" is also frequently used for level-banded (parallel-banded) agates, but in proper usage it refers to color pattern not band structure. Onyx, as a descriptive term, has also been incorrectly applied to parallel-banded varieties of alabaster, marble, calcite, obsidian, and opal, and misleadingly to materials with contorted banding, such as "cave onyx" and "Mexican onyx".