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Fireworks. Jellyfish. Galaxies. Lightning. Opal’s shifting play of kaleidoscopic colors is unlike any other gem.

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Most opals are cut as cabochons; this one from Ethiopia weighs 20.86 ct.

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Unique Beauty

The play-of-color displays the entire spectrum over a light gray background.

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Bright flashes of color skip across this stone’s surface.

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Hidden Beauty

Opal forms as irregular nodules or seams in volcanic or sedimentary rocks.

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Rare Phenomena

Play-of-color results when light interacts with orderly stacks of spheres.

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This opal nodule displays the promise of intense play-of-color.

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About Opal

About Opal

Because opal has the colors of other gems, the Romans thought it was the most precious and powerful of all. The Bedouins believed that opals contained lightning and fell from the sky during thunderstorms. When Australia’s mines began to produce opals commercially in the 1890s, it quickly became the world’s primary source for this October birthstone.

Opal Description

Opal History and Lore

Birthstones & Anniversaries

Opal is an October birthstone.

0.2 Microns

Grids of silica spheres 0.2 microns in size create red play-of-color flashes.

20% Water

Opal contains up to 20% water trapped in its silica structure.


The novel “Anne of Geierstein” gave opal a reputation of being unlucky.


  • Mineral: Hydrated Silica
  • Chemistry: SiO2•nH2O
  • Color: All colors
  • Refractive index: 1.37-1.47
  • Birefringence: None
  • Specific gravity: 2.15 (+0.08, -0.90)
  • Mohs Hardness: 5 to 6.5


There are a number of processes used to alter the color, apparent clarity, or improve the durability of gems.

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Some gemstones have synthetic counterparts that have essentially the same chemical, physical, and optical properties, but are grown by man in a laboratory.

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Any gem can be imitated—sometimes by manmade materials or by natural materials chosen by man to impersonate a particular gem.

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gem love

Why We Love This Gemstone


When opal formed, silica gel filled crevices in rock. As water evaporates, the silica is deposited in the form of tiny spheres.

Interaction with Light

Opal’s flashing play-of-color is caused by diffraction of light by silica spheres stacked like tiny Ping-Pong balls in a box.


Opal’s arrays of silica spheres form a fantastic variety of patterns and colors. No two opals are exactly alike.

Quality Factors

Play-of-color, intensity, and pattern are important value factors.


quality factors

Opal’s spectacular play-of-color can display all the colors of the rainbow.


quality factors

Experts expect different levels of clarity for different types of opals.


quality factors

Fine opals are often cut into irregular shapes that keep as much play-of-color as possible.

Carat Weight

quality factors

Opal has relatively low density so even larger sizes can be comfortable to wear.

Opal Quality Factors: The Comprehensive Guide


Explore sources, gemological research, and the role of gems in history.

 Hand holding tube-shaped boulder opal

Queensland Opal Fields: Home of the Unique Australian Boulder Opal

Tao Hsu, Andrew Lucas, and Vincent Pardieu , Jan 24, 2017 Read Article
Color bars on top and sides of 3,019 ct rough opal

The Challenges of Cutting a Large Gem Opal Rough

Theodore Grussing , Aug 3, 2016 Read Article
Underground opal mining in Koroit

Splendor in the Outback: A Visit to Australia's Opal Fields

Tao Hsu, Andrew Lucas, and Vincent Pardieu , Feb 1, 2016 Read Article
Spinel’s visible spectrum

New Deposit of Black Opal from Ethiopia

Lore Kiefert, Pierre Hardy, Tewodros Sintayehu, Begosew Abate, and Girma Woldetinsae , Jan 30, 2015 Read Article