A lab-grown sapphire is not a fake. It is real corundum, with the same crystal structure, the same chemical formula, the same 9 on the Mohs scale and the same refractive index as a stone pulled out of the ground in Sri Lanka. That identity is exactly what makes separation such a demanding piece of gemology — and why a certificate from a competent lab matters more for corundum than for almost any other gem.
Why Standard Gem Tests Cannot Separate Them
The classical toolkit of gem identification — refractive index, specific gravity, hardness, dichroism — is built to distinguish one species from another. It will tell you instantly that a stone is corundum rather than spinel, glass, or cubic zirconia. What it cannot tell you is where that corundum came into being, because a laboratory crystal and a geological one produce identical readings.
This is a very different problem from separating a sapphire from a simulant. A blue glass or a synthetic spinel gives itself away in seconds under a refractometer. A flame-fusion synthetic sapphire does not. Separation therefore depends almost entirely on growth evidence: the microscopic traces left behind by how the crystal formed, and the trace-element signature it inherited from its environment.
The Three Ways Synthetic Corundum Is Grown
Understanding the evidence starts with understanding the manufacturing. Nearly all synthetic sapphire and ruby on the market comes from one of three processes, and each leaves its own fingerprint.
- Flame fusion (Verneuil): The oldest and cheapest method, dating to the 1890s. Powdered alumina is dropped through an oxyhydrogen flame and solidifies onto a rotating boule. Fast, high-volume, and the source of most inexpensive synthetic ruby.
- Flux growth: Alumina is dissolved in a molten flux and allowed to crystallise slowly over weeks or months. Far more convincing, because the slow growth mimics natural conditions.
- Hydrothermal growth: Crystals are grown from a hot aqueous solution under pressure, on a seed plate. The most difficult of the three to identify, and the one that most closely reproduces natural growth chemistry.
Cost roughly tracks difficulty. A flame-fusion ruby is worth a few dollars a carat; a well-grown flux synthetic is expensive to produce and, unsurprisingly, is the type most often encountered in situations where someone hoped it would not be questioned.
Inclusions: The Gemologist's First Line of Evidence
Under a microscope, growth history becomes visible. Flame-fusion material typically shows curved growth striae — gently arcing colour banding that has no equivalent in nature, where corundum grows in straight, angular, hexagonal bands following the crystal's own geometry. Alongside the curved banding, spherical or teardrop gas bubbles are common, another artefact of a molten droplet process.
Flux synthetics tell a different story. They contain residual flux trapped during growth, appearing as wispy, fingerprint-like veils or twisted feathers, often with a slightly whitish or golden cast. These can superficially resemble the healed fractures found in natural stones, and distinguishing the two is where practical experience matters. Hydrothermal stones may show a visible seed plate or a distinctive chevron-shaped growth pattern near the base of the crystal.
Natural corundum, by contrast, carries the debris of its geological environment: rutile silk, zircon crystals with tension halos, negative crystals filled with fluid, and angular colour zoning. Those inclusions are the same features gemologists use for origin determination and heat detection, which is why one thorough microscopic examination often answers several questions at once.
When the Microscope Is Not Enough
A cleanly grown flux or hydrothermal stone, well cut to remove the most telling zones, can be effectively inclusion-free. At that point the major laboratories turn to instrumentation.
FTIR spectroscopy detects water-related absorption features that are characteristic of hydrothermal growth and absent in most natural stones. UV-Vis-NIR spectroscopy maps the absorption behaviour of chromophores such as iron, titanium and chromium; natural blue sapphire almost always shows iron-related bands that flame-fusion material lacks entirely, because there is no iron in the feedstock. LA-ICP-MS measures trace elements down to parts per million, and the ratios between gallium, magnesium, iron and titanium are the single most decisive line of evidence available — synthetic corundum is grown from purified alumina and simply does not carry the messy geological cocktail that natural crystals do.
In combination, these techniques close the question conclusively. This is also why a report from a laboratory that runs full spectroscopic analysis carries more weight than a basic identification card, particularly at higher price points.
What This Means for Buyers
Synthetic corundum is legitimate material with legitimate uses, and disclosed honestly it is not a problem. The problem is undisclosed material entering the supply chain, which happens most often through informal channels, tourist markets, and mixed parcels bought without documentation.
The practical protection is straightforward: for any meaningful purchase, buy stones accompanied by a report from a recognised laboratory, and buy from sellers who state treatment and origin in writing. At Thai Gems, every stone is described with its treatment status disclosed, and certification is available across our inventory. Sourcing directly from Bangkok — the point where the majority of the world's corundum is cut and graded — removes several of the layers where undisclosed material tends to slip in.
Browse our current selection of natural sapphires and natural rubies, or contact us for trade pricing, certification, and custom orders.