Opal is a gem made of silica that water left in cracks of underground rock. Precious opal is made of tiny spheres stacked in a grid, and these diffract light into play-of-color.
Properties
| Refractive index | 1.37-1.47 (Mexican/fire opal usually 1.42 to 1.43, as low as 1.37) |
|---|---|
| Birefringence | None (singly refractive) |
| Optic character | Singly refractive (amorphous) |
| Dispersion | Not given by our sources |
| Specific gravity | 2.15 (+0.08, -0.90) (GIA), i.e. 1.25 to 2.23 including porous opal; most opals 1.9 to 2.23 (IGS), 1.98 to 2.25 (gemdat); fire opal about 1.95 |
| Hardness (Mohs) | 5 to 6.5 |
| Crystal system | Amorphous |
| Pleochroism | None |
| Absorption spectrum | Most opals show no useful spectrum |
Published values. They change a little from stone to stone. Our tools do not model opal yet. Sources: GIA · International Gem Society · Gemdat · LibreTexts Gemology
In more detail
Types include white, black, boulder, fire and crystal opal, named for their background color and transparency. Opal can be impregnated with oil, wax or plastic, or given sugar or smoke treatment. It has very poor to fair toughness, and strong light or sudden heat can cause crazing or fractures. Clean it only with warm, soapy water.
Related terms
- Assembled stone: An assembled stone is made of two or more pieces joined together, usually with glue or by fusing. A doublet has two parts; a triplet has three, or two parts with a colored layer between them.
- Toughness: Toughness is a gem's resistance to chipping and breaking. It differs from hardness, which is resistance to scratching.
- Stability: Stability is a gem's resistance to damage from heat, light and chemicals.
- Cabochon: A cabochon is a gem polished to a smooth, convex dome, with no facets. It is one of the oldest and simplest cuts.
- Resin filling: Resin filling is filling a gem's fissures or cavities with a resin, often one that hardens, to hide them.