Heat, irradiation, and coatings

Dye is the treatment that gets discussed, and it is not the only one. Heat alters colour by rearranging lattice defects, irradiation creates those defects deliberately, and coatings add colour or iridescence on the surface without touching the material underneath. All three are ordinary trade practice, they differ enormously in how detectable and how durable they are, and only one of them is easy to spot.

Heating

What it does. Heat changes colour by destroying or creating colour centres, by altering the oxidation state of impurity elements, and sometimes by driving off water or dissolving internal features. It works on colour that comes from a lattice defect and not on colour that comes from inclusions, which is a useful rule for predicting which materials are heat-treated.

Where it appears in these materials. Amethyst heated to citrine is the standard example. Carnelian and agate are heated to deepen red-brown tones, a practice thousands of years old. Some agates are heated after a sugar or iron treatment.

How stable it is. Generally very. A heat-induced colour has already survived the highest temperature the material will ever see, so it does not fade back. Among treatments, heating is the one least likely to change the object over its lifetime.

How detectable it is. Poorly. There may be effects on internal features — altered inclusions, distinctive stress fractures around them — and there may be nothing at all. This is a laboratory question and often an unresolved one, which is part of why heating is so widely undisclosed: the trade cannot always tell either.

Irradiation

What it does. Exposure to a radiation source creates lattice defects, producing colour centres. Natural radiation in the ground does this over geological time; a facility does it in hours. Colourless quartz irradiated becomes smoky quartz; the same principle produces blue topaz from colourless material.

How stable it is. Variable, and this is the practical concern. Some irradiation-induced colours are stable indefinitely; others fade with light or moderate heat, sometimes quickly. A stone whose colour faded on a windowsill may have been irradiated, though natural colour centres fade too and fading does not establish treatment.

On residual activity. Irradiated gem material is subject to regulation in several markets, with requirements to hold treated material until any induced activity has decayed to background before release. This is a real regulatory framework rather than a rumour, and it applies to the treatment methods capable of inducing activity — not all of them are.

How detectable it is. Generally not, outside a laboratory, and sometimes not there either.

Coatings and films

This is the detectable one, and the most various.

Vapour-deposited metallic films. Quartz points and slices are coated in a vacuum chamber with a thin layer of metal — titanium, gold, and others — producing intense iridescence by thin-film interference. Sold under a range of “aura” names. The colour is a surface phenomenon and the material underneath is ordinary quartz.

Lacquer, paint and backing. Colour applied to the back of a translucent piece, or a coloured film behind a hollow-backed cabochon, so that the body appears coloured. Old technique, sometimes called foiling or backing.

Surface dye and coloured wax. A layer of colour held at the surface by a wax or polymer film rather than driven into the material. Common on porous stone.

Plain wax and oil. No colour added; fills microscopic surface pits, improves polish and deepens apparent tone. Traditional and accepted in the jade trade, and the mildest treatment there is.

How to spot a coating. All coatings share one weakness: they are thin, and the boundary is at the surface.

• Colour that stops abruptly at a scratch or chip, with different material visible underneath. This is conclusive.
• Iridescence that behaves like a film — shifting with viewing angle in the way a soap bubble does rather than in the way a mineral sheen does.
• Wear patterns at high-contact points and edges where the film has thinned or gone.
• Colour concentrated in recesses and carved detail, where a liquid pooled and dried.
• A slight difference in lustre between the coated surface and any exposed break.

WHAT YOU CAN TELL — treated beyond dyeing

  · Colour stops at a chip, pale material beneath
      → surface coating. Conclusive
  · Iridescent film shifting with angle, wearing
      at edges → vapour-deposited coating
  · Colour pooled in carved recesses
      → applied liquid colour
  · Greasy feel, softens with warmth
      → wax; surface treatment only
  · Colour faded unevenly with light exposure
      → unstable colour centre; treated or natural
  · Whether a stone has been heated or irradiated
      → NOT DETERMINABLE at home, and frequently
        not determinable in a laboratory either

Why the disclosure record is so uneven

Three reasons, and they are structural rather than moral.

Detectability drives disclosure. A treatment that cannot be detected cannot be policed, so it becomes assumed rather than declared. Heating is the clearest case: it is so universal in some materials that the trade treats untreated as the claim requiring proof.

Antiquity confers acceptance. Practices centuries old — waxing jade, heating agate — are considered part of normal processing rather than treatments, and are described that way in trade custom. The line is drawn by history, not by physics.

Structural intervention is treated differently, and rightly. The reason polymer impregnation is classified separately from waxing is that it rebuilds the material and then degrades, and that distinction is the one that has real consequences for the object over time.

Where a treatment is undisclosed further down a supply chain, the usual cause is information loss rather than intent — though the direction of the errors is never random.

The limit

Coatings you can often catch, because the evidence is at the surface and a chip exposes it. Wax you can sometimes infer. Dye you can frequently see.

Heat and irradiation you cannot. Both leave the material’s composition, hardness and density unchanged, both may leave no internal trace, and the laboratory answer is sometimes “indeterminate” rather than a verdict. This is worth knowing chiefly so that an absence of visible treatment is not mistaken for evidence of an untreated stone — what home examination establishes is narrower than the confident version of this subject implies, and the gap is largest exactly here.