The vast majority of citrine in the contemporary trade is produced by heat treatment of natural amethyst. Natural untreated citrine is comparatively rare, and the trade has converged on heat-treated material as the practical commercial source for most yellow to orange-yellow quartz used in jewellery. The treatment is generally regarded as stable and acceptable in the trade, although disclosure of the treatment is required.

Process

Amethyst is the violet to purple variety of quartz, with the colour produced by an iron-related colour centre that requires both iron impurity and natural irradiation to form. When amethyst is heated to temperatures between approximately 400 and 500 degrees Celsius, the colour centre is destroyed and the iron impurity rearranges into a configuration that produces a yellow to orange-yellow colour. The transformation is permanent and stable, although re-irradiation can in principle restore the original amethyst colour in some material.

The depth of colour and the specific hue produced by heating depend on the iron content of the parent amethyst, the heating temperature, and the duration. Brazilian amethyst from the Rio Grande do Sul deposits is the principal commercial source, and produces a range of citrine colours from pale lemon-yellow through deep orange-brown. The deepest orange-brown material is sometimes marketed as Madeira citrine, after the Portuguese fortified wine.

Some amethyst, when heated, produces a green colour known as prasiolite or green amethyst rather than yellow. The green colour is also stable, though some prasiolite material can fade in prolonged sunlight exposure. Prasiolite is itself produced from specific amethyst deposits, principally in Minas Gerais in Brazil, with green-producing iron-related colour centres distinct from the typical violet centres.

Disclosure and trade conventions

The CIBJO Coloured Stone Book and the Federal Trade Commission Jewelry Guides require disclosure of heat treatment in citrine. The disclosure may be in the form of a treatment indication on a laboratory report, in trade documentation accompanying the stone, or in retail point-of-sale information. The treatment is so widespread in the citrine trade that the default trade assumption is that any commercially available citrine has been heat-treated unless specifically documented as natural.

Natural untreated citrine is found principally in small deposits in Brazil, Madagascar, Bolivia (where it occurs as ametrine alongside amethyst in the same crystal), and Russia. The natural untreated material is generally pale to medium yellow, often with a slight greenish tone, and rarely reaches the deep saturated orange of the heat-treated Madeira material. Connoisseurs of natural quartz often prefer the more subtle colour of unheated material, but the price premium for unheated citrine is modest because of the difficulty of definitively distinguishing heated from unheated citrine in many cases.

Identification

Distinguishing heat-treated from natural citrine is difficult and not always possible. The principal diagnostic features are subtle. Natural citrine often shows pleochroism in pale yellow, while heat-treated material may show more saturated and uniform colour. Inclusion patterns can sometimes be diagnostic; some heat-treated material retains visible relict colour zoning from the parent amethyst, although in well-treated stones this is not visible. Spectroscopic measurements may distinguish the two types in some cases through differences in the iron-related absorption features, but the distinction is not always clear-cut.

The trade convention has been to assume heat treatment as the default and to charge a modest premium for documented untreated material. The major laboratories generally do not provide treatment-status reports specifically for citrine in the commercial price range, although high-value or claimed-natural specimens can be examined.

Position in the market

Citrine occupies the middle and lower price ranges of the coloured-stone market, with retail prices typically in the range of tens to low hundreds of dollars per carat for fine-quality faceted material. The November birthstone designation in many traditions has supported a stable demand for citrine in commercial jewellery. The principal cutting centres for commercial citrine are Idar-Oberstein in Germany, Bangkok, Jaipur, and several Brazilian centres.

The trade does not generally regard heat treatment as a defect in citrine; the stability of the treated colour and the long-standing convention of accepting heat treatment in this material have placed citrine alongside heat-treated tanzanite, heat-treated aquamarine, and certain heat-treated tourmalines as a category in which heat treatment is the trade norm and is acceptable provided it is disclosed.