A flawless stone is a stone that tells you nothing. Inclusions — the foreign crystals, fluid pockets, needles, fractures and growth patterns locked inside a gem — are a record of where and how it formed, and no two environments leave the same record. The microscope is the instrument that reads them, and for the two questions that matter most to buyers — natural or synthetic, treated or not — it is the instrument that decides.
What inclusions are
As a crystal grows, it can trap other minerals that were present, pockets of the fluid it grew from, gas bubbles, and traces of its own growth history in the form of zoning and twinning. Later events — heating, stress, healing of fractures — add more. Everything trapped inside is an inclusion, and each has a shape, a colour and a relationship to the host that reflects the process that made it.
In the earth, crystals grow slowly over geological time from mineral-rich fluids or melts under pressure. In a factory, they grow in days or weeks from a flame, a flux or a pressurised solution. The two environments leave different fingerprints, and that is the whole basis of natural-versus-synthetic determination.
Inclusions of natural stones
Mineral crystals. Angular, well-formed crystals of other minerals: rutile, zircon, calcite, mica, pyrite, spinel. Each species has its typical guests; zircon crystals with stress haloes are classic in sapphire, pyrite in emerald, garnet in some diamonds.
Silk. Fine needles of rutile, arranged in intersecting sets at 60° in corundum, giving the sheen that makes star rubies and sapphires. Intact silk is strong evidence of no or low heating, because high temperatures dissolve it.
Fingerprints and feathers. Healed fractures where the crystal re-grew across a break, trapping a plane of tiny fluid droplets that looks like a fingerprint. Very common in corundum, spinel and quartz.
Fluid inclusions. Cavities holding liquid, sometimes with a gas bubble and a small crystal — the three-phase inclusions characteristic of Colombian emerald.
Colour zoning. Straight, angular bands of colour following the crystal's growth faces. Angular zoning is natural; curved zoning is not.
Twinning and growth tubes. Planes and tubes reflecting the crystal's structure.
Inclusions of synthetic stones
Curved striations. Flame-fusion synthetics (Verneuil) grow as a boule from a flame, layer on layer, and the layers are curved. Curved growth lines and curved colour banding do not occur in nature.
Gas bubbles. Round, sometimes elongated or in trails, typical of flame-fusion material and of glass.
Flux residues. Flux-grown synthetics (ruby, emerald, spinel) trap wisps and veils of the flux — the solvent they grew in — that look like fingerprints but are denser, often white or coloured, and sometimes accompanied by fragments of the platinum crucible as tiny metallic platelets.
Chevron and seed features. Hydrothermal synthetics show characteristic zigzag or chevron growth zoning and may show the seed plate they grew from.
Absence of everything. Some synthetics are so clean that the absence of any natural inclusion is itself suspicious, and advanced tests are used. How synthetic gems are grown describes each process and its signatures.
Inclusions that reveal treatment
Heat. Silk partly or wholly dissolved into dotted lines; crystals surrounded by discoid tension fractures where the inclusion expanded more than the host; altered "snowball" appearance of some crystals. Heat treatment in ruby and sapphire.
Glass filling. Fractures filled with glass that show trapped gas bubbles, a blue or orange "flash" at the boundary where the RI of glass and host differ, and a crazed or crackled texture. Glass-filled ruby.
Oil and resin. Fissures in emerald showing a flash of colour, trapped bubbles, and sometimes a whitish residue where the filler has dried. Emerald oiling and resin filling.
Dye. Colour concentrated in fractures and along grain boundaries rather than through the body of the stone.
Diffusion. Colour that follows the facet outlines rather than the crystal structure, visible when the stone is immersed. Diffusion-treated sapphire.
How the examination is done
The stone is examined at 10× with a loupe and then at 10× to 60× under a stereo microscope with darkfield illumination (light from the side against a dark background, which makes inclusions bright), brightfield, and often immersion in a liquid of similar RI to reduce surface reflections. The examiner catalogues what is present and forms a conclusion from the combination. A single feature rarely decides; a pattern does.
Frequently asked questions
Are inclusions bad?
For clarity grading, yes — they reduce the stone's transparency. For identification, they are the evidence. A clean stone is harder to prove natural than an included one.
Can inclusions be added to a synthetic to fool a lab?
Attempts exist — synthetics have been grown with deliberately included natural-looking features — and laboratories know them. The full pattern of a stone's inclusions is very hard to counterfeit.
Can I see inclusions with a loupe?
Many, yes, at 10×. Recognising what they mean takes training and reference material. A loupe tells you there is something inside; the laboratory tells you what.
Do all natural stones have inclusions?
Almost all have something at high magnification. Truly clean natural stones exist and are valuable; they are also the ones a laboratory examines most carefully.
Does an inclusion affect durability?
Some do. A large fracture reaching the surface, or a crystal under tension, can be a weakness. A report may note such features.