A synthetic gemstone is grown, not manufactured. It is a real crystal of the same material as the natural stone, produced by giving the right chemistry the right conditions to crystallise. There are three broad ways of doing that, and each writes its process into the crystal in a way a microscope can read.
Flame fusion (Verneuil)
The oldest method, industrialised in 1902 by Auguste Verneuil for ruby, and still the source of most cheap synthetic corundum and spinel. Powdered aluminium oxide with a colouring agent is dropped through an oxy-hydrogen flame at over 2,000 °C. The powder melts and falls onto a rotating pedestal, where it solidifies layer by layer into a carrot-shaped boule several centimetres long. The boule is split and cut.
Signatures. Because the boule builds up from the outside of a rotating drop, growth lines and colour banding are curved — an arc pattern that never occurs in nature, where growth follows flat crystal faces. Round gas bubbles, sometimes in clouds or lines, are common. The material is usually very clean apart from these. Flame-fusion ruby often fluoresces very strongly under UV, more than most natural material.
Products. Ruby, all colours of sapphire, star ruby and sapphire (with added rutile), spinel in many colours (blue synthetic spinel is a common aquamarine and sapphire imitation), rutile and strontium titanate.
Flux growth
Developed for emerald in the 1930s and for ruby in the 1950s and 60s. The gem's components are dissolved in a molten flux — a solvent such as lithium molybdate or lead fluoride — in a platinum crucible at around 1,000–1,300 °C. As the solution slowly cools over weeks or months, crystals grow, with natural crystal faces, from seed crystals or spontaneously.
Signatures. Because the crystal grew from a solution with flat faces, its zoning is angular like natural material — which is why flux synthetics are harder to identify. The giveaway is the flux itself: wispy, veil-like or net-like residues of trapped solvent, often white, yellowish or coloured, resembling fingerprints but denser and with a characteristic texture. Platinum platelets — tiny, triangular or hexagonal metallic flakes from the crucible — are diagnostic where found. Growth may show chevron or "comet tail" patterns.
Products. Emerald (Chatham, Gilson and others), ruby (Chatham, Kashan, Ramaura, Knischka), sapphire, alexandrite, spinel.
Hydrothermal growth
A closer imitation of nature. The components are dissolved in water at high temperature and pressure — typically 300–600 °C, hundreds of atmospheres — inside a sealed steel autoclave, with a temperature gradient that dissolves nutrient at one end and deposits crystal on seeds at the other. This is how virtually all quartz for electronics is grown, and how some emerald, ruby and beryl are produced.
Signatures. Chevron or zigzag growth zoning in emerald and beryl, from the way the crystal grows on a seed cut at an angle to the natural faces. Seed plate sometimes visible as a flat plane in the crystal. Nail-head spicules — tapered tubes with a crystal at the tip — in some hydrothermal emerald. Hydrothermal quartz can show fine cobweb-like or "bread-crumb" inclusions and a characteristic planar growth zoning parallel to the seed.
Products. Quartz in all colours (amethyst, citrine, ametrine, blue and green quartz), emerald (Biron, Tairus, Regency), ruby and sapphire (Tairus), beryl in colours.
Other methods
Pulling (Czochralski) — a seed is dipped into a melt and withdrawn slowly, drawing a crystal; used for alexandrite, corundum and garnet-structured materials like YAG. Curved striations similar to flame fusion but finer. Floating zone and skull melting (for cubic zirconia). HPHT and CVD for diamond, covered in Lab-grown diamonds.
What this means for identification
The refractometer and specific gravity say corundum or beryl or quartz and stop there — natural and synthetic read the same. The microscope is where the distinction is made, by finding features that belong to one origin and not the other. Curved lines and bubbles: flame fusion. Flux veils and platinum: flux. Chevrons and seed planes: hydrothermal. Angular zoning, natural mineral crystals, silk, fingerprints: natural. The combination is rarely ambiguous in the hands of an experienced examiner, though very clean flux and hydrothermal stones can require advanced instruments. Inclusions: the fingerprints inside a gemstone.
Frequently asked questions
Which synthetics are hardest to detect?
Very clean flux-grown ruby and emerald and some hydrothermal material, because their growth zoning is angular like natural stones. Laboratories rely on the flux residues, platinum and specific growth features, and on spectroscopy where needed.
Are synthetics sold openly?
Yes, and legitimately, as laboratory-grown or created stones at a fraction of natural prices. The problem is exclusively mis-selling as natural.
Does a synthetic have the same hardness and durability?
Yes. It is the same material. A synthetic sapphire wears exactly like a natural one.
Why are flame-fusion stones so cheap?
The process is fast — a boule grows in hours — and the raw materials are inexpensive. Flux and hydrothermal growth take weeks to months and cost correspondingly more, though still far less than mining.
Can a synthetic be treated?
Yes. Synthetic corundum can be heated or diffused, and synthetic emerald can be oiled. Treatment and origin are separate questions.