If you are shopping for opal, working with it as a maker, or trying to identify a stone you already own, the first question is almost always the same: is this natural or is it lab-grown?

It is a fair question, and the answer matters. Not because one is "real" and one isn't, but because the two materials behave differently. They have different durability, different price points, different uses, and different physical properties that make each one the right tool for some jobs and the wrong one for others.

This guide walks through how to tell them apart. The visual cues, the structural tests, what gemologists actually look for, and what makes each material the right choice in the first place.

First, Let's Clear Up the Language

A lot of confusion in this space comes from sloppy terminology. Before you can tell two things apart, you need to know what they actually are.

Natural opal is opal that formed in the ground over millions of years. It is made of microscopic silica spheres stacked in a lattice, with water filling the spaces between the spheres. That sphere arrangement is what creates the play-of-color you see.

Synthetic opal is lab-grown opal with the same fundamental structure. Silica spheres, same lattice, same diffraction physics, same play-of-color. The difference is in what fills the spaces between the spheres and how the spheres themselves were arranged. Some synthetic opals replace the water with a polymer stabilizer for added durability. Others are entirely resin-free and contain pure silica, much like natural opal but produced under controlled conditions.

Opal-structured composites are materials that produce real play-of-color through real silica spheres and Bragg Diffraction, but contain a higher polymer content than gemological standards require for the "synthetic" classification. They are not the same as natural or synthetic opal, but they are also not in the same category as glass or plastic imitations. They are their own thing, and a useful one for certain applications.

Pure imitations are materials designed to look like opal without sharing its silica-sphere structure at all. Foiled glass, Slocum stone, and plastic resins with iridescent flakes fall here. These do not produce play-of-color through diffraction the way real opal does.

The distinction between these categories matters more than most buyers realize, and it is the source of most of the confusion floating around online.

The Visual Tests You Can Do at Home

You do not need a gemology lab to spot most differences. A jeweler's loupe (10x magnification) or even a clear macro photo from your phone is enough for the majority of identifications.

Look for columnar structure from the side

This is the single most reliable visual indicator. Many lab-grown opals are produced by allowing silica spheres to settle vertically over time. That growth process leaves a visible columnar pattern when you view the stone from the side. You will see distinct vertical bands or stripes of color running through the material.

Natural opal almost never displays this structure. It forms in irregular pockets and seams in the earth, so the play-of-color appears in random, chaotic arrangements that do not repeat in clean columns. If you can see vertical color bands on the side of a stone, it is almost certainly lab-grown.

Check the face for "lizard skin" or "chicken wire" patterns

Under magnification, many lab-grown opals show a repeating, geometric pattern across the face of the stone. Color patches form in roughly uniform polygonal cells with thin dark outlines between them. Gemologists call this the "lizard skin" or "chicken wire" effect, and it is one of the classic tells.

Natural opal patches are messier. Sizes vary, edges are uneven, and the mosaic looks organic rather than tiled. Neither pattern is better than the other. They are just different, and they tell you what you are working with.

Watch how the color flashes move

Tilt the stone slowly under a single light source. Lab-grown opal often "synchronizes" because the lattice is so regular. Large zones flip from one color to another together. Natural opal flashes are more independent. Different patches catch the light at different angles, and the overall effect feels less choreographed.

Look for inclusions and imperfections

Natural opal frequently contains tiny inclusions: micro-fractures, host-rock fragments, internal cloudiness, or small areas where the play-of-color fades into "potch" (common opal without fire). Lab-grown opal tends to be cleaner because it is produced in controlled conditions. A stone with zero visible flaws under magnification is often a sign of lab origin, while a stone with visible internal complexity is more likely natural.

The Structural and Physical Tests

If visual inspection is not enough, a few simple physical tests can give you more information.

What Lab Testing Reveals

For a definitive answer, a certified gemological lab can run analytical tests that go beyond visual inspection.

• Specific gravity measurements distinguish natural opal (typically around 2.0 to 2.2) from polymer-stabilized lab opal (often 1.7 to 1.8) and resin-free lab opal (closer to natural values).
• Infrared spectroscopy identifies the water content in natural opal versus the polymer signatures in stabilized lab opal. The 1,730 cm⁻¹ absorption band is a classic indicator of resin content.
• X-ray fluorescence can detect trace elements like chlorine and zirconium that distinguish certain lab-grown opals (some Gilson-process opals were known to contain zirconium oxide spheres mixed in with the silica).

These tests are the gold standard, and any high-value opal should come with documentation from a reputable lab.

Identification Quick Reference

If you only remember a handful of differences, these are the ones that matter most.

A Quick Word on Gilson Opal

If you have researched lab-grown opal at all, you have probably come across Gilson opal. Pierre Gilson developed the first commercially successful resin-free synthetic opal in 1974. His process produced a 100% silica material with no water and no polymer, distinguishing it from polymer-stabilized lab opals that came later.

Gilson stopped producing opal decades ago. The original Gilson patents and processes were sold in 1980, and while related products have appeared since, original Gilson opal is no longer manufactured.

Our Glass Opal™ is the most similar to natural opal of any product we offer. It has a higher silica content than typical lab opal and contains no water, similarly to Gilson Opal that is no longer produced. It is our own proprietary trade name for our 100% silica, resin-free crushed opal product line.

Choosing the Right Material for the Job

Knowing what you are working with is not just about identification. It changes how you should approach the material.

Natural opal is beautiful and unique. Every stone is one of a kind. It is also fragile, often expensive, and frequently inconsistent. Drilling natural opal beads larger than 6mm is difficult because the material is prone to cracking. Ethiopian Welo absorbs water and can craze over time. Color matching across a production run is nearly impossible. None of this makes natural opal worse. It just makes it the right tool for some jobs and the wrong tool for others.

Lab-grown opal solves different problems depending on the type. At The Opal Dealer, we make three:

Each material is the right answer for different jobs. The "best" opal depends entirely on what you are making.

Frequently Asked Questions