Opal
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A hydrated amorphous silica, opal is celebrated for play-of-color, a shifting spectral display of red, green, blue, and gold produced as light diffracts through ordered arrays of microscopic silica spheres. Australia yields roughly ninety percent of the world's precious opal, including white opal from Coober Pedy, black opal from Lightning Ridge, and boulder opal from Queensland, with further sources in Ethiopia and Mexico. Lacking crystalline structure, it measures 5.0 to 6.5 on the Mohs scale. Revered since antiquity and the birthstone for October, opal is classed as semi-precious in the budget tier, yet fine black opal with red fire ranks among the most valuable colored stones per carat.
Physical & Optical Properties
Category
Semi-Precious Gems
Mohs Hardness
5.0 – 6.5
Specific Gravity
1.98 – 2.25
Refractive Index
1.37 – 1.52
Crystal System
Amorphous
Cleavage
None
Fracture
Conchoidal,Splintery
Color
white
Pleochroism
Absent
Mohs Hardness
| Mohs | Reference mineral | Opal |
|---|---|---|
| 1 | Talc | |
| 2 | Gypsum | |
| 3 | Calcite | |
| 4 | Fluorite | |
| 5 | Apatite | ● |
| 6 | Orthoclase | ● |
| 7 | Quartz | |
| 8 | Topaz | |
| 9 | Corundum | |
| 10 | Diamond |
Crystal System
Value & Market
Price Tier
Budget (< $50/ct)
Where to Buy Opal
Blue Nile
Fine jewelry retailer
James Allen
Gemstone jewelry
Brilliant Earth
Ethically sourced gems
Amazon
Wide selection
Links may earn a commission at no extra cost to you.
Cultural Significance
Birthstone Month
10 (modern)
About Opal
オパールは水和非晶質シリカ（SiO₂・nH₂O）で、石内のシリカ球の規則正しい配列による光の回折により赤・緑・青・金色が幻想的に変化する遊色（プレイ・オブ・カラー）で知られます。オーストラリアが世界の貴重オパールの約90%を産出し、クーバーペディの白色オパール、ライトニングリッジの黒色オパール、クイーンズランドのボルダーオパールが代表的で、エチオピアやメキシコも重要産地です。古代から珍重され10月の誕生石でもある赤い炎のある上質な黒色オパールは、カラット当たりの価値が最も高い有色宝石の一つです。
Science FYI Family
Frequently Asked Questions
What is Opal?
オパールは水和非晶質シリカ（SiO₂・nH₂O）で、石内のシリカ球の規則正しい配列による光の回折により赤・緑・青・金色が幻想的に変化する遊色（プレイ・オブ・カラー）で知られます。オーストラリアが世界の貴重オパールの約90%を産出し、クーバーペディの白色オパール、ライトニングリッジの黒色オパール、クイーンズランドのボルダーオパールが代表的で、エチオピアやメキシコも重要産地です。古代から珍重され10月の誕生石でもある赤い炎のある上質な黒色オパールは、カラット当たりの価値が最も高い有色宝石の一つです。
What is the hardness of Opal?
Opal has a Mohs hardness of 5.0 to 6.5, making it moderate for use in jewelry.
What color is Opal?
Opal is primarily White.
Where is Opal found?
Opal is found in Lightning Ridge (Australia).
How much does Opal cost?
Opal falls into the Budget (< $50/ct) price range. Prices vary based on color, clarity, cut, and origin.