Opal – Crystal Wiki
The mineraloid with fascinating play of color – every structure looks different
Opal (SiO2·nH2O)
Properties & Characteristics
Scientific Data
- Chemical Formula
- SiO2·nH2O
- Crystal System
- Amorphous (no crystal system)
- Mohs Hardness
- 5.5–6.5
- Density
- 1.9–2.3 g/cm3
Physical Properties
- Color
- White, gray, black, transparent; sometimes with strong play of color
- Luster
- Waxy to vitreous
- Transparency
- Transparent to opaque
- Special Feature
- Precious opal shows interference play of color
What is Opal?
Opal is a hydrated silicon dioxide without long-range crystalline order. Precious opals in particular are coveted worldwide for their play of color.
Formation
Opal forms through the precipitation of silica-rich solutions in cracks and cavities. This results in various types such as boulder or hyalite opal.
Deposits & Market
Australia is one of the most important opal regions, along with Ethiopia and Mexico.
Quality Characteristics
The intensity and pattern of the play of color, body color, cut, and stability determine the market value.
Care & Handling
Protect from impact, strong heat, and extreme changes in humidity. Store and clean gently.
Frequently Asked Questions
Is opal delicate?
Yes, opal is more delicate than many classic gemstones.
What distinguishes precious opal from common opal?
Precious opal shows a distinct play of color, common opal does not.
Can opal crack?
Cracking can occur under unfavorable tension and climatic conditions.