How to Tell Real Gemstones from Fake: Opal & Moonstone Guide

Imitations — not stones at all

These are pieces of glass, resin, or plastic made to look like natural gemstones. A well-known example is "opalite," a milky man-made glass that is sometimes sold as moonstone or opal. Because it is not a mineral, it should never be labelled a "natural stone." The problem is not the material itself — it is when it is sold as something it is not.

Synthetics — genuine in composition, born in a lab

Synthetic stones share essentially the same chemical composition as their natural counterparts, but are created artificially. In the world of opal, the "Kyoto Opal" developed by Kyocera in Japan is a well-known example. A synthetic stone is an entirely legitimate product — as long as it is clearly labelled as synthetic. The trouble begins when a lab-grown stone is quietly sold as natural.

Treated stones — natural, but altered

These are natural stones that have been dyed, heated, or assembled from layers. Treatments are widespread in the gem trade, and many are accepted under industry standards. However, whether a stone has been treated directly affects its value, which is why it should be disclosed at the point of sale. The words "natural stone" alone tell you nothing about treatment — and that is precisely what makes choosing gemstones difficult.

Is the rainbow shimmer a little too regular?

Opal's signature charm — its play-of-color — is created by microscopic silica spheres inside the stone that diffract light. In natural opal, this color pattern is irregular, shifting in character as the viewing angle changes. Synthetic opal, by contrast, can show color patches that look too orderly, and under magnification may reveal a regular, scale-like pattern often described as "lizard skin."

Another thing to watch for is the assembled stone. Doublets and triplets — a very thin slice of natural opal glued to dark backing or glass — can reveal themselves when viewed from the side: the boundary between layers appears as an unnaturally straight line. If the design covers the sides and back of the stone completely and you cannot check, the safest approach is simply to ask the seller about the stone's specifications.

Dyeing is not a crime — it is a cost decision

The first thing to understand is that there are clear economic reasons why dyed stones are used. Pale, lower-grade stones can be sourced for far less than richly colored ones. Dye them, and they resemble higher-grade material — while also providing the uniformly colored lots that mass production depends on. For supplying inexpensive, vividly colored accessories at scale, it is an entirely rational method.

That said, dyeing comes with characteristics worth knowing. Porous stones such as turquoise, howlite, and agate absorb dye easily and have long been common candidates for this treatment — but their color can fade over time, or transfer onto skin and clothing through contact with sweat and moisture. There are also known cases of inexpensive white stones being dyed and sold under the name of a more valuable gem. A "natural" stone where every single piece is exactly the same color — that is one of the signs.

Dyeing itself, when properly disclosed, is not dishonest. The problem arises when a dyed stone is sold with nothing but the label "natural stone," because the buyer pays the price of an untreated stone while expecting its value.