- Color: wide spectrum of multicolor splashes with the body tones from colorless/milky to deep black.
- Hardness: 5.5 - 6.5
- Refractive index: 1.42 - 1.47
- Density: 1.95 - 2.35
- Chemical composition: SIO2*nH2O
- Crystal structure: amorphous
- Origins: Australia (the major source), Ethiopia, Mexico, Honduras, Guatemala, Brazil, USA, Canada, Madagascar, Tanzania, Czech, Slovakia.
An opal is an amorphous form of silica (in the quartz group of minerals). Precious opals from Australia or Ethiopia have an intense color play in a wide spectral range. Opals contain layers of microspheres of silica, which produce the effect of light interference. The Color grading system for opals is based on two scales: Spectrum & Body Tone. Black opals with intense red color splashes are considered the most valuable gemstones. Opals are not treated.