Emerald
Emerald is a gemstone and a variety of the mineral beryl (Be3Al2(SiO3)6) colored green by trace amounts of chromium or sometimes vanadium. Beryl has a hardness of 7.5–8 on the Mohs scale. Most emeralds have much material trapped inside during the gem's formation, so their toughness (resistance to breakage) is classified as generally poor. Emerald is a cyclosilicate.
| Emerald | |
|---|---|
| General | |
| Category | Beryl variety |
| Formula (repeating unit) | Be3Al2(SiO3)6 |
| Crystal system | Hexagonal (6/m 2/m 2/m) Space group: P6/mсc |
| Space group | (6/m 2/m 2/m) – dihexagonal dipyramidal |
| Unit cell | a = 9.21 Å, c = 9.19 Å; Z = 2 |
| Identification | |
| Formula mass | 537.50 |
| Color | Bluish green to green |
| Crystal habit | Massive to well Crystalline |
| Cleavage | Imperfect on the [0001] |
| Fracture | Conchoidal |
| Mohs scale hardness | 7.5–8 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent to opaque |
| Specific gravity | Average 2.76 |
| Optical properties | Uniaxial (−) |
| Refractive index | nω = 1.564–1.595, nε = 1.568–1.602 |
| Birefringence | δ = 0.0040–0.0070 |
| Ultraviolet fluorescence | None (some fracture-filling materials used to improve emerald's clarity do fluoresce, but the stone itself does not) |
| References | |
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