Opal

Q: What is Opal?

Opal is a hydrated amorphous silica (SiO₂·nH₂O) celebrated for play-of-color—a spectral shifting display of red, green, blue, and gold caused by diffraction of light through ordered arrays of silica spheres within the stone. Australia produces approximately 90% of the world's precious opal, includin

Q: What is the hardness of Opal?

Opal has a Mohs hardness of 5.0 to 6.5, making it moderate for use in jewelry.

Q: Where is Opal found?

Opal is found in Lightning Ridge (Australia).

Q: How much does Opal cost?

Opal falls into the Budget (< $50/ct) price range. Prices vary based on color, clarity, cut, and origin.

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 hardness of Opal shown against the ten reference minerals, from talc (1) to diamond (10).

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

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Cultural Significance

Birthstone Month 10 (modern)

About Opal

Opal is a hydrated amorphous silica (SiO₂·nH₂O) celebrated for play-of-color—a spectral shifting display of red, green, blue, and gold caused by diffraction of light through ordered arrays of silica spheres within the stone. Australia produces approximately 90% of the world's precious opal, including white opal from Coober Pedy, black opal from Lightning Ridge, and boulder opal from Queensland; significant sources also include Ethiopia and Mexico. Revered since antiquity and October's birthstone, fine black opal with red fire is among the most valuable colored gemstones per carat.