Gem Decoded : Opal

OPAL

Species : Hydrated Silica

Opal is a mesmerizing hydrated amorphous form of silica (SiO₂·nH₂O), known for dazzling play-of-color and unique internal structure.

What sets opal apart is its signature play-of-color, a stunning optical phenomenon caused by the diffraction of light through tiny, orderly silica spheres within the stone. It creates a vivid spectrum of shifting hues comprising fiery reds, vibrant oranges, electric blues, and lush greens. The finest opals showcase bright, broad flashes of color evenly distributed across the gem, captivating anyone who gazes upon them.

Australia reigns supreme as the world's premier source of precious opal, particularly the legendary Lightning Ridge black opals. They are popular for deep, dark backgrounds that dramatically enhance the vibrant color play. Ethiopia's Wollo and Shewa regions have also emerged as important producers. They offer opals with striking patterns and vibrant colors at more accessible price points. Mexico is the contributor of fire opals, glowing warmly in shades of red, orange, and yellow, prized for their intense, transparent colors.

Being the birthstone for October, it's believed to carry the power of amplification and transformation. Ancient civilizations treasured this gem as a symbol of hope and prophecy. Opal has been associated with creativity, intuition, and emotional healing, making it a gem of brilliance and inner harmony.

In this article, we delve deep into the scientific marvels of opal and what makes each stone so uniquely captivating. For those eager to learn more, we've gone the extra mile and covered some technical FAQs.

General Information

Classification

Optical Properties

Physical Properties

Chemistry

The 4Cs of Amethyst

Color

Opals are renowned for their mesmerizing play-of-color. It's a shifting, rainbow-like iridescence across the surface in flashes of red, orange, yellow, green, blue, and violet. The body color, or background hue, can range from white, light gray, and pale yellow (common in white opals) to deep black (in prized black opals), as well as transparent to semi-transparent in crystal opals. The most coveted opals, especially those from Lightning Ridge, Australia, display intense, multi-hued flashes against a dark body tone, making the play-of-color more vivid. Fire opals, primarily from Mexico, are a distinct variety that displays a bright, transparent to translucent, orange to red body color, sometimes with or without play-of-color. The brilliance, pattern, and range of colors determine the value, with broad, vibrant flashes being the most desirable.

Cut

Opal is a hydrated silica known for its mesmerizing play-of-color, caused by light interacting with microscopic silica spheres. With a refractive index between 1.37 and 1.52, opal is typically cut as a cabochon to best showcase its shifting flashes of color. Unlike faceted gems, opal's beauty lies in its surface pattern and light play, which vary with angle and lighting. Freeform and asymmetrical cuts are also popular, especially for boulder and crystal opals. Its unique look makes opal a striking and artistic choice for jewelry.

Clarity

Opals can range from transparent to opaque, and clarity is judged based on both transparency and the presence of inclusions or cracks. In crystal opals, higher transparency and a clean internal structure are preferred to allow the play-of-color to shine without obstruction. In solid opals, the focus is on color clarity and the absence of crazing – tiny, spiderweb-like cracks that can develop due to dryness or age. Potch (common opal without play-of-color) may be present in some stones, which can decrease value unless artfully used in boulder opals. The finest opals are free of visible inclusions or structural flaws and display uninterrupted, vibrant play-of-color across the surface.

Carat

Opal’s specific gravity ranges from 1.98 to 2.25, depending on type and water content. Unlike many gemstones, opals form in a wide range of sizes—from tiny accent stones to large specimens used for carvings or display. Fine-quality opals over 5 carats often command higher per-carat prices due to more vivid play-of-color and broader surface area to display it. However, very large opals can see a dip in per-carat value, especially if the size makes them less practical for wearable jewelry. Demand tends to favor well-proportioned stones with vibrant, balanced color play.

Types of Opal

Pricing as per quality:

History

Opal’s history stretches back at least 6,000 years, with the earliest known artifacts discovered in Kenya and believed to have originated from Ethiopia. The Greeks thought opals could grant the gift of prophecy and protect against disease, while the Romans saw them as symbols of love and hope. The Roman scholar Pliny the Elder, writing in 75 AD, marveled at opal’s kaleidoscopic colors, comparing them to the richest hues used by painters and the fiery brilliance of other precious stones.

The Sanskrit word upala-meaning “precious stone”-is the root of the word “opal,” which traveled through Greek (opallios) and Latin (opalus) before becoming the name we use today.

For centuries, the only known source of opal in Europe was in what is now Slovakia, making it extremely rare and valuable, often reserved for royalty. In the Middle Ages, opal was considered a stone of great luck, believed to possess all the virtues of every colored gemstone it resembled.

Arabic folklore claimed opals fell from the sky in flashes of lightning. Indigenous Australians, whose land would later become the world’s main opal source, assigned the stone cosmic significance, tying it to their Dreamtime stories.

Everything changed in the late 19th century when vast opal fields were discovered in Australia. The first major find was in Queensland in the 1860s, followed by the iconic Lightning Ridge discovery in New South Wales in the late 1880s. By the early 20th century, Australia dominated the opal market, and today, it supplies about 95% of the world’s opals.

Frequently Asked Questions

Q: Are all opals play-of-color opals?

No, only “precious opals” show play-of-color. Common opals lack this effect and appear opaque or milky without colorful flashes.

Q: How much water is in opal?

Opal typically contains 3-21% water by weight, which influences its stability and durability.

Q: Why do opals sometimes crack or craze?

Loss of water due to drying or sudden temperature changes can cause internal stress, leading to cracking or crazing.

Q: What are the main types of opal?

The main types include black opal, white opal, crystal opal, fire opal, and boulder opal, each distinguished by background color and transparency.

Q: What makes black opal special?

Black opal has a dark body tone that creates a dramatic contrast, enhancing the vibrancy of the play-of-color.

Q: Can opal be treated or enhanced?

Yes, treatments include dyeing, impregnation with resins, and sugar-acid treatments to improve color or stability.

Q: Is opal isotropic or anisotropic?

Opal is isotropic. Hence, its optical properties are the same in all directions.

Q: What causes the body color of opal?

Body color arises from impurities, the density of silica spheres, and the amount of water within the stone.

Q: What is a “harlequin” pattern in opal?

The harlequin pattern features large, distinct patches of play-of-color arranged in a checkerboard-like pattern. It’s highly prized by collectors.