- Color: from light yellow to orange, brownish red, with milky haze.
- Hardness: 5.5 - 6.0
- Refractive index: 1.37 - 1.43
- Density: 2.15
- Chemical composition: SIO2nH2O
- Crystal structure: Amorphous
- Origins: Major source is Mexico, however similar stones are found in Brazil and USA.
Mexican opals are an amorphous form of silica (quartz group of minerals), and are the national gemstone of Mexico. These stones are fragile, and while they may be used for pendants and earrings, Mexican opals are not recommended for rings due to their insufficient hardness. Mexican opals are not treated.