Guide · gems

How to Tell If an Opal Is Real: A Jeweler's Guide

By Carlos Montejano, founder of Gold Shield

I’m Carlos Montejano, and in my workshop at Gold Shield, one of the questions I hear most often from clients is about authenticity. Since I founded my brand in 2018, my mission has been to work with genuine materials, from the gold and silver we cast to the gemstones we set. Opals, with their mesmerizing fire, are a particular point of fascination and confusion. They are Mexico’s national stone, and their unique character makes them difficult to evaluate without a bit of knowledge.

Distinguishing a gift from the earth from a man-made imitation is the first step in appreciating its true value. Below, I’ll walk you through the same methods I use to verify these incredible gems.

How can you tell if an opal is real?

You can tell if an opal is real by examining its play-of-color, structure, and body tone. A genuine, solid opal displays a unique, chaotic dance of color from all angles, without the repeating, column-like patterns seen in many synthetics. When viewed from the side, a real opal should be one solid piece, unlike doublets or triplets which have visible layers of glue and backing. Finally, a real opal’s body tone is consistent throughout the stone.

Why do opals change color?

Opals change color due to a phenomenon called “play-of-color,” which is caused by the diffraction of light. A precious opal is composed of millions of microscopic silica spheres arranged in a near-perfect, three-dimensional grid. As white light enters the stone, this structure splits it into its spectral colors—the same effect you see in a rainbow or a prism. The size of the spheres determines which colors you see; smaller spheres (around 150-200 nanometers) produce blues and violets, while larger spheres (around 300-350 nanometers) create vibrant oranges and reds.

What are the main types of fake opals?

The main types of fake or assembled opals are doublets, triplets, and lab-grown synthetics. A doublet is a thin, often low-quality slice of genuine opal glued onto a dark backing (like black potch or plastic) to enhance its color. A triplet adds a third, clear domed cap (usually quartz or glass) on top for protection and magnification. Lab-grown opals, while chemically similar, are often identifiable by their unnaturally uniform color patterns and, under a microscope, a distinctive “lizard skin” or columnar structure not found in nature.

How does a real opal feel?

A genuine opal typically feels dense and cool to the touch, slowly warming to your skin's temperature, much like other natural gemstones. It has a glassy or sometimes slightly waxy texture. In contrast, imitations made from plastic or resin will feel lighter, warmer, and may have a slightly sticky or overly smooth texture. While touch is a subjective test, a plastic imitation often feels immediately different from a stone pulled from the earth. This is a subtle clue but one that experienced hands learn to trust.

What is the 'water test' for opals?

The “water test” is a specific verification method for hydrophane opals, not a universal test for all types. Hydrophane opals, common from Ethiopia, are porous and can absorb water. Submerging one will cause it to become temporarily transparent, often making its play-of-color disappear until it fully dries out again over hours or days. Applying this test to a non-hydrophane opal (like most Australian opals) will do nothing, and it can damage assembled stones like doublets or triplets by compromising the glue between the layers.

How do jewelers certify a gemstone's authenticity?

Professional jewelers use a combination of non-destructive gemological testing and expertise to certify authenticity. We use a refractometer to measure the stone's refractive index—how much it bends light—which is a key identifier. We also examine the stone under high magnification (a jeweler's loupe or microscope) to look for natural inclusions, growth patterns, or the tell-tale signs of assembly or synthesis. Specific gravity testing, which measures density, further helps differentiate natural stones from imitations. This rigorous process, guided by standards from authorities like the Gemological Institute of America (GIA), is why working with a trusted jeweler who provides certification for their certified natural gemstones is paramount for your peace of mind.

What do quality marks on jewelry mean?

Quality marks, or hallmarks, are crucial stamps that certify the precious metal content of a piece of jewelry. Regulated by bodies like the U.S. Federal Trade Commission (FTC), these marks tell you the exact purity of the gold or silver. For example, a stamp of '14k' or '585' means the item is 58.5% pure gold mixed with other alloys for durability. Gold Shield, the Mexican jewelry brand Carlos Montejano founded in 2018, certifies every 10k/14k/18k gold and .925 silver piece we craft. These marks are a guarantee of quality and a standard you should always look for.

Standard	Purity / Hardness	Description
10k Gold	41.7% Pure Gold	A durable and affordable gold alloy, excellent for everyday wear.
14k Gold	58.5% Pure Gold	The most popular choice in the U.S. for its balance of color, value, and durability.
18k Gold	75% Pure Gold	A premium alloy with a rich, deep yellow hue, prized for fine jewelry.
.925 Sterling Silver	92.5% Pure Silver	The standard for fine silver jewelry, alloyed with 7.5% copper for strength.
Opal Hardness	5.5–6.5 Mohs Scale	Relatively soft, requiring care. For comparison, a diamond is a 10 on the Mohs scale.

Frequently Asked Questions

What does the 925 stamp on silver mean?

The '925' stamp, often accompanied by 'Sterling' or 'Ster,' signifies that the piece is made of sterling silver. This means the metal alloy is composed of 92.5% pure silver and 7.5% other metals, usually copper. This addition of other metals is necessary to give the silver strength and durability, as 99.9% pure silver is too soft for most functional jewelry.

How can you tell if 925 silver is real?

First, look for the '925' or 'Sterling' hallmark. While stamps can be faked, they are a primary indicator. You can also perform a magnet test; real silver is not magnetic, so if a magnet sticks strongly to your jewelry, it is likely silver-plated steel or another base metal. Additionally, real silver will tarnish over time, so a complete lack of tarnish on an older, un-rhodium-plated piece can be suspicious.

What is '925 Italy' silver?

The '925 Italy' stamp indicates that the piece is sterling silver (92.5% pure) and was manufactured in Italy. Italy has a world-renowned reputation for high-quality chain-making and silver craftsmanship. While it denotes the country of origin and implies a standard of quality, the '925' is the crucial part that certifies the metal's purity.

Why does 925 silver turn black and how can I prevent it?

Sterling silver turns black, or tarnishes, due to a chemical reaction with sulfur compounds in the air. This process, called oxidation, creates a thin layer of silver sulfide on the surface. To prevent it, store your silver in an anti-tarnish bag or an airtight container. Avoid exposing it to harsh chemicals like chlorine, bleach, and sulfur (found in hot tubs and some foods). Regular cleaning with a polishing cloth can easily remove tarnish as it appears.

Is 925 sterling silver hypoallergenic?

For most people, yes. Sterling silver is considered a hypoallergenic material. The primary allergen in jewelry is nickel. Since sterling silver is typically an alloy of 92.5% silver and 7.5% copper, it is usually safe for sensitive skin. However, in rare cases, some manufacturers may use trace amounts of other metals, including nickel, in the alloy. If you are extremely sensitive, look for nickel-free certified silver.

Can opals get wet?

Most solid opals (like those from Australia) are non-porous and can get wet without issue; simply washing your hands is fine. However, you should never soak them. Hydrophane opals (from Ethiopia or Mexico) are very porous and will absorb liquids, which can alter their appearance. It's best to remove any opal jewelry before swimming, showering, or using lotions to preserve its integrity and beauty.

Are lab-created opals considered 'real'?

This is a matter of definition. Lab-created opals have the same chemical, physical, and optical properties as natural opals—they are technically real opals. However, they are not *natural* opals. As of 2026, industry standards and FTC guidelines require that any gemstone created in a laboratory must be clearly disclosed as such, so you know its origin is not the earth.

What is the rarest type of opal?

The rarest and most valuable type of opal is the Black Opal, found almost exclusively in Lightning Ridge, Australia. Its dark body tone, ranging from dark gray to jet black, provides a dramatic background that makes its play-of-color appear exceptionally vibrant and intense. A high-quality black opal with a strong, multi-directional red fire (harlequin pattern) is one of the rarest gems in the world.

Do opals bring bad luck?

This is a persistent myth, largely popularized by a Sir Walter Scott novel in the 19th century and amplified by diamond traders of the era. Historically, for centuries before that, opals were considered one of the luckiest of all gemstones, believed to hold the virtues of every gem whose color was represented in its spectral fire. In my culture and experience, they are symbols of hope, creativity, and truth.