Most of the confusion in the coloured-stone market comes down to three words that are used loosely in shops and precisely in laboratories: natural, synthetic and simulant. Once you understand the distinction, a great deal of marketing language becomes transparent — including the phrases that are designed to blur it.
Natural
A natural gemstone formed in the earth, without human involvement, over geological time. It may have been cut and polished, which every gem is, and it may have been treated — heated, oiled, filled, irradiated — which is a separate question dealt with below. But the crystal itself grew in rock.
Natural stones carry the evidence of that growth inside them: mineral inclusions from the surrounding rock, healed fractures, colour zoning that follows crystal faces, and the small imperfections that come from growing slowly in an uncontrolled environment.
Synthetic
A synthetic gemstone has the same chemical composition and crystal structure as its natural counterpart, but was grown by people in a laboratory or factory. Synthetic ruby is aluminium oxide coloured by chromium, exactly like natural ruby. It has the same hardness, the same refractive index, the same specific gravity. Under the standard bench tests it is ruby.
The difference is where and how fast it grew, and that shows in the inclusions. Flame-fusion synthetics (the cheapest, made since the early 1900s) show curved growth striations and round gas bubbles. Flux-grown synthetics show wispy flux residues and chevron-shaped growth. Hydrothermal synthetics show characteristic growth zoning near the seed. None of these features occur in nature. How synthetic gems are grown describes each process.
Legitimate trade terms for synthetic stones include synthetic, laboratory-grown, laboratory-created and man-made. Terms designed to mislead include "cultured" (borrowed from pearls, where it means something quite different), "created gemstone" without the word laboratory, and any regional or brand name presented as if it were a variety.
Simulant
A simulant — also called an imitation — is any material that looks like a gemstone but is not that gemstone. Red glass is a ruby simulant. Cubic zirconia is a diamond simulant. Blue synthetic spinel is a sapphire simulant (it is genuinely synthetic spinel, but it is sold to imitate sapphire). Dyed quartz is an emerald simulant.
A simulant may itself be natural (garnet-topped doublets, natural spinel sold as ruby), synthetic (CZ, synthetic spinel) or manufactured glass. What makes it a simulant is not the material but the intent: it is standing in for something else.
Simulants are the easiest of the three to identify because their physical properties do not match the stone they imitate. Glass "ruby" has a refractive index around 1.5–1.7 and a single reading; real ruby (natural or synthetic) reads 1.762–1.770 with two readings. The polariscope, refractometer and specific gravity separate simulants in minutes.
Treated
Treatment is a fourth word that cuts across the first three. A natural stone can be treated; so can a synthetic. Treatment means any process after mining that alters the stone's appearance: heating to improve colour, filling fractures with glass or resin, diffusing colour into the surface, irradiating, dyeing, coating.
Some treatments are so standard that the trade assumes them (most blue sapphire is heated; most emerald is oiled), but every treatment must be disclosed, because it affects value, durability and care. A heated sapphire is still natural sapphire. A glass-filled ruby is a natural ruby with so much glass in it that some laboratories classify it as a composite. Heat treatment in ruby and sapphire and Glass-filled ruby explain the two most important cases.
How the laboratory separates them
| Question | Instrument | What decides it |
|---|---|---|
| Is it the species claimed, or a simulant? | Refractometer, specific gravity, polariscope | RI and SG match the species or they do not |
| Natural or synthetic? | Microscope, sometimes UV and spectroscope | Growth features and inclusions characteristic of nature or of the factory |
| Treated or untreated? | Microscope, UV, spectroscope; advanced spectroscopy for some heat questions | Altered inclusions, filler in fractures, colour concentration in cracks, surface-conformal colour |
The order matters. Species first, because a simulant needs no further testing. Then natural versus synthetic. Then treatment.
What a buyer is entitled to
The answer to all three questions, in writing, before paying. A laboratory report should state the species, whether the stone is natural or synthetic, and whether any treatment was detected. If a report says "ruby" without saying "natural" or "synthetic", it has not answered the most valuable question. If it says "natural ruby" without mentioning treatment, ask whether treatment was tested for and what the finding was.
Price is the practical consequence. Natural untreated ruby of fine colour is among the most expensive materials on earth per gram. Synthetic ruby of identical colour costs a small fraction. Glass-filled ruby costs less still. The three can look alike in a shop window.
Frequently asked questions
Is a synthetic gemstone fake?
Not in the sense of being a different material — synthetic ruby is chemically ruby. It becomes a deception only when sold as natural, or at a natural price. Sold honestly as laboratory-grown, it is a legitimate product.
Can a laboratory always tell synthetic from natural?
For the vast majority of stones, yes, using the microscope. A few very clean synthetics — some hydrothermal emerald, some flux ruby — need advanced instruments. A cautious laboratory says when a determination needed more than the standard bench.
What does "lab-certified natural" mean if the stone is treated?
It means the crystal grew in the earth and has since been altered. Both facts should be on the report. "Natural" describes origin; "treated" describes what happened afterwards. Neither word cancels the other.
Are simulants always cheap?
Usually, but not always. A natural spinel sold as ruby is a simulant in that transaction, and spinel is a valuable stone in its own right. The problem is the mislabelling, not the material.
Does "created" mean synthetic?
If it means anything, it means synthetic — but sellers use it precisely because it sounds like something else. Ask directly: was this stone grown in a laboratory?