Amethyst, citrine, and heat treatment

Amethyst is purple quartz, and the purple comes from a defect in the crystal structure involving iron rather than from an included pigment. Defects of that kind can be undone with heat, and heating amethyst turns it yellow to orange — which is to say, into citrine. The bulk of commercial citrine is heat-treated amethyst, this is entirely standard, and it explains a great deal about the colour range you see in quartz objects.

Colour by defect

Amethyst is quartz — silicon dioxide, Mohs 7, specific gravity about 2.65 — coloured purple by a mechanism that is not a pigment. Iron substitutes into the lattice, natural irradiation over geological time promotes it to an unusual oxidation state, and the resulting colour centre absorbs light. The colour is a property of a structural defect, not of a foreign substance dispersed in the material.

Two things follow directly, and both are observable.

Colour is often zoned. Amethyst commonly shows angular colour banding — sectors and stripes of stronger and weaker purple following the crystal’s growth zones. Natural amethyst is frequently uneven in exactly this structured way, which is why perfectly uniform colour is not the norm.

Colour is heat-sensitive and light-sensitive. Because the colour centre can be destroyed, sustained heat removes the purple, and prolonged exposure to strong light can fade it over long periods. This is a real property of natural material: a faded amethyst has not necessarily been treated with anything.

Smoky quartz works the same way, its brown coming from an aluminium-related colour centre produced by natural irradiation. Colourless quartz can be irradiated artificially to produce it, which is the reverse operation of heating.

What heating does

Heat amethyst and the purple centre breaks down. Depending on the material and the temperature, the result is yellow, orange, reddish-brown, colourless, or — in specific material heated to particular temperatures — the green quartz sold as prasiolite. Some amethyst heats to a yellow-and-purple bicolour, sold as ametrine, and this occurs naturally too.

Citrine is yellow to orange quartz. It exists naturally and is uncommon; the great majority of the citrine on the market is heat-treated amethyst. This is long-established, generally accepted in the trade, and typically undisclosed at the retail end not out of deception but because the treatment is assumed by anyone in the business.

The treated colour is stable. Unlike the purple it replaced, the yellow does not readily revert, so a heat-treated citrine is not a degrading product in the way that polymer-impregnated material is. This is the important distinction within the treatment category: heating changes a colour permanently and leaves the material structurally intact, while impregnation changes the structure and ages.

Distinguishing heated from unheated is difficult and belongs firmly with heat, irradiation and coating as a laboratory question. Some heated material shows characteristic effects on internal features; much shows nothing usable.

Synthetic amethyst

Quartz is grown industrially in very large quantities, and coloured synthetic quartz — including amethyst — is produced commercially. A synthetic amethyst is amethyst: same composition, same hardness, same refractive index, same density. Nothing about being synthetic makes it a lesser material physically.

Identification is a specialist matter. The standard approach examines a particular twinning pattern that almost all natural amethyst displays and that much synthetic material lacks, together with growth features and inclusions, under a microscope with the stone immersed. This is difficult work, it is not always conclusive, and it is completely unavailable to a non-specialist with a polished opaque object in hand.

The practical position for a tool made of purple quartz: synthetic is plausible, undetectable by you, and physically identical in use.

What you can establish

WHAT YOU CAN TELL — a purple or yellow quartz tool

  · Steel will not mark it, it marks glass
      → hardness near 7; consistent with quartz
  · Angular colour zoning in bands or sectors
      → natural growth zoning; good sign
  · Perfectly uniform saturated colour
      → suspect dye or glass, not proof
  · Bubbles or flow swirls under a loupe → glass
  · Colour webbed along cracks → dyed material
  · Heated versus unheated, natural versus synthetic
      → NOT DETERMINABLE at home. Both are
        laboratory questions and neither is always
        answerable there

The zoning observation is the useful one. A dye enters from outside and follows cracks and grain boundaries; a growth zone is geometric and follows the crystal’s own former faces, producing straight edges and angular sectors. The two look quite different once you know to look for the difference between a web and a wedge.

Purple things that are not amethyst

Dyed quartzite, purple glass and purple-dyed agate all appear in the tool market. Fluorite, which occurs in fine purple, is soft — around 4 on Mohs, with cleavage in four directions — and makes a genuinely fragile object; a purple stone that a steel point marks and that shows flat cleavage faces is not amethyst. Charoite and lepidolite are also purple and both softer than quartz.

The hardness test is unusually informative in this colour range, because amethyst is quartz and most purple substitutes are considerably softer.

Depth of colour is worth one further note. Amethyst spans very pale lilac to deep purple, and the pale material is abundant while the deep material is not. A large, evenly deep purple object at a low price is therefore describing an unusual combination of size, colour and cost, and the ordinary resolutions are dye, glass, or a coating on pale material. None of that establishes anything on its own; it establishes which observation to make next, which is generally the loupe.

The limit

For purple and yellow quartz, the achievable findings are hardness, structural class and colour distribution. Those separate quartz from glass and from softer purple minerals, and they distinguish webbed dye from natural zoning. That is a real result and it is the kind of result home testing gives.

Everything else here is out of reach, and unusually so. Whether the yellow was purple last year, and whether the crystal grew in a vessel or in a vein, are questions requiring immersion microscopy by a trained examiner. Both are extremely common in the market, and neither changes what the object is made of: quartz, hardness 7, brittle, and coloured by a defect that somebody may or may not have arranged.