Andesine is a member of the plagioclase feldspar series, occupying the intermediate compositional range between albite (sodium aluminium silicate) and anorthite (calcium aluminium silicate), with an anorthite content of approximately 30 to 50 mole percent. In its natural, untreated state it is typically colourless, pale yellow, or grey — an unremarkable mineral of considerable geological abundance but limited gem significance. Its entry into the fine jewellery trade in the early 2000s, however, was anything but unremarkable. A wave of vivid red-to-red-orange feldspar, marketed variously as andesine, andesine-labradorite, or Tibetan andesine, appeared on the international gem market around 2003 and rapidly attracted both commercial enthusiasm and gemmological suspicion. Subsequent investigation by the Gemological Institute of America, the American Gem Trade Association, and independent researchers established that the overwhelming majority of this red material had been subjected to copper diffusion treatment — a fact that had been systematically withheld from buyers. The episode remains one of the most thoroughly documented cases of undisclosed treatment in the modern gem trade.

Mineralogy and Physical Properties

Andesine belongs to the triclinic crystal system and is a member of the plagioclase group, which forms a continuous solid-solution series from pure albite (Ab₁₀₀) to pure anorthite (An₁₀₀). Andesine occupies the Ab₇₀An₃₀ to Ab₅₀An₅₀ range. Its physical properties are consistent with other plagioclase feldspars:

• Crystal system: Triclinic
• Hardness: 6 to 6.5 on the Mohs scale
• Specific gravity: Approximately 2.65 to 2.69
• Refractive indices: Approximately 1.543 to 1.563 (biaxial negative)
• Cleavage: Perfect in two directions at nearly right angles, characteristic of the feldspar group
• Lustre: Vitreous
• Transparency: Transparent to translucent

Natural andesine may occasionally display adularescence or a weak schiller effect, though these phenomena are far more pronounced in other plagioclase members such as labradorite and moonstone. In its natural gem-quality form, andesine is a minor species of limited commercial importance. The geological environments in which it forms include intermediate igneous rocks such as andesite (from which it takes its name), diorite, and certain metamorphic assemblages.

The Red Andesine Controversy: Origins and Timeline

The red feldspar that entered the market from approximately 2003 onwards was initially presented as a newly discovered natural gemstone from Tibet and, subsequently, from the Democratic Republic of Congo and Mongolia. The material displayed an attractive red to red-orange colour with good transparency and was offered in faceted form at prices consistent with a mid-range coloured stone. Trade publications and gem shows in the United States and Europe featured it prominently, and it was embraced by a segment of the jewellery industry before any systematic scientific scrutiny had been conducted.

Scepticism arose relatively quickly among experienced gemmologists. The colour was unusually saturated for a feldspar, the geographic provenance claims were inconsistent and difficult to verify, and the supply appeared to be remarkably steady — a characteristic more consistent with a treated material produced in volume than with a newly discovered natural deposit. By 2008 and 2009, formal investigations were underway at several major laboratories.

The AGTA Gemological Testing Center published findings indicating that the red colour in the material it examined was the result of copper diffusion treatment. The GIA's research, published in Gems & Gemology, corroborated this conclusion through a combination of analytical techniques including laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), which detected anomalously high copper concentrations in the red zones of the stones. Crucially, the copper was found to be concentrated near the surface of the stones, consistent with diffusion from an external source rather than incorporation during crystal growth. Spectroscopic analysis further supported the diffusion hypothesis.

Independent researchers and other gem laboratories reached substantially the same conclusions. The treatment process, as reconstructed from the evidence, involved heating yellow or colourless feldspar in a copper-rich environment at elevated temperatures, causing copper ions to migrate into the crystal lattice and produce the red coloration. This is analogous in principle — though different in chemistry — to the beryllium diffusion treatment applied to corundum, which had itself caused significant trade disruption in the early 2000s.

Gemmological Detection of Treatment

Identifying copper-diffused andesine requires access to advanced analytical instrumentation; standard gemmological testing alone is generally insufficient to confirm or exclude the treatment. The principal detection methods established by laboratory investigation include:

• LA-ICP-MS: The most definitive technique. Copper concentrations in treated material are dramatically elevated relative to untreated feldspar, and the spatial distribution — highest at the surface, diminishing toward the interior — is diagnostic of diffusion.
• Energy-dispersive X-ray fluorescence (EDXRF): Can detect elevated copper but is less sensitive than LA-ICP-MS and may miss low-level treatment.
• Spectroscopy: Absorption features associated with copper can be identified in the visible and near-infrared spectrum, though interpretation requires expertise.
• Surface examination: Under magnification, treated stones may show colour concentration near facet edges, surface pitting consistent with high-temperature processing, or other surface characteristics inconsistent with natural crystal growth.
• Refractive index and specific gravity: These standard tests are of limited value in distinguishing treated from untreated material, as the treatment does not substantially alter the bulk physical properties of the feldspar host.

The GIA and AGTA both concluded that disclosure of the copper diffusion treatment was essential and that stones sold without such disclosure were being misrepresented. The AGTA's position, consistent with its broader treatment disclosure policy, was that copper diffusion treatment must be disclosed at every level of the trade.

The Question of Natural Red Feldspar

A central and genuinely difficult question raised by the controversy was whether any naturally coloured red feldspar exists at all. The answer, based on the available scientific literature, is that natural red feldspar is extraordinarily rare and that the vast majority of red feldspar encountered in the gem trade — including essentially all of the material marketed as Tibetan andesine — is treated.

Oregon sunstone, a labradorite-andesine feldspar from the Plush and Spectrum Mine deposits in Oregon, United States, does occur naturally in red and red-orange colours. The colour in Oregon sunstone is attributed to copper in the form of native copper platelets or dissolved copper within the crystal, incorporated during the original magmatic crystallisation of the host basalt. This is a fundamentally different situation from diffusion treatment: the copper is primary, distributed throughout the stone, and present at concentrations consistent with natural geological processes. Oregon sunstone is the most credible and well-documented example of naturally red feldspar in the gem trade, and it is subject to independent laboratory verification.

Some researchers and traders argued during the controversy that small quantities of the Chinese and Mongolian material might be naturally coloured. While this cannot be categorically excluded, the weight of laboratory evidence — particularly the surface-concentrated copper distributions found in the overwhelming majority of examined stones — indicated that natural colour was, at best, vanishingly rare in the commercial supply. The GIA's published research found no confirmed examples of naturally coloured red andesine from the Chinese or Mongolian sources among the stones it examined.

Geographic Sources and Provenance Claims

The provenance narrative attached to red andesine shifted several times as scrutiny intensified, which itself was regarded by investigators as a warning sign. Initial marketing emphasised Tibet as the source, lending the material an aura of remoteness and exoticism. When this claim proved difficult to substantiate — Tibet is not a documented source of gem-quality feldspar — the origin was variously amended to Inner Mongolia, Sichuan Province in China, and the Democratic Republic of Congo.

The DRC claim was particularly significant because the Kivu region of the DRC does contain feldspar occurrences, and some researchers initially considered the possibility that naturally coloured material might originate there. However, laboratory analysis of material purportedly from the DRC yielded the same copper diffusion signatures as the Chinese material, and the provenance claims were not independently verified through geological fieldwork or chain-of-custody documentation.

By contrast, the Oregon sunstone deposits in Lake County, Oregon, are thoroughly documented, have been mined commercially for decades, and produce material whose natural colour has been confirmed by multiple independent laboratories. The Ponderosa Mine and Spectrum Mine are among the named producing localities. Oregon sunstone occupies a distinct and uncontroversial position in the feldspar gem trade, entirely separate from the red andesine controversy.

Trade Impact and Disclosure Standards

The red andesine controversy had measurable consequences for the segment of the trade that had embraced the material. Retailers who had purchased and sold red andesine without disclosure found themselves in difficult positions with customers; some faced requests for refunds or restitution. The episode reinforced the importance of laboratory testing and chain-of-custody documentation for any novel gem material entering the market, and it contributed to broader industry discussions about treatment disclosure standards.

The AGTA's treatment disclosure guidelines, which require disclosure of any treatment that affects value or durability, clearly encompass copper diffusion treatment. The GIA's position, as expressed in its published research, was equally unambiguous: the treatment is significant, it is not detectable by standard gemmological means without advanced instrumentation, and it must be disclosed. Any red andesine or red feldspar offered without a laboratory report from a recognised testing facility should be regarded with caution.

In the years following the peak of the controversy, red andesine largely receded from mainstream gem shows and trade publications, though it continues to appear in certain market segments, sometimes without adequate disclosure. The episode is now routinely cited in gemmological education as a case study in the risks of novel gem materials and the necessity of independent laboratory verification.

Relationship to Other Feldspars

Understanding andesine's place within the broader feldspar group is useful for contextualising both its mineralogy and its market position. The plagioclase series, of which andesine is a member, ranges from albite through oligoclase, andesine, labradorite, bytownite, and anorthite with increasing calcium content. Gem-quality members of this series include:

• Moonstone: Primarily orthoclase or albite-oligoclase intergrowths, prized for adularescence.
• Labradorite: An₅₀–An₇₀, celebrated for its iridescent labradorescence.
• Oregon sunstone: Compositionally in the labradorite-andesine range, valued for its natural copper-derived colours and aventurescence.
• Peristerite: An albite-oligoclase intergrowth displaying a bluish adularescence.

Andesine in its natural, untreated form is not a significant gem species in its own right. Its commercial identity in the gem trade is almost entirely defined by the treated red material and the controversy surrounding it — a situation that distinguishes it from virtually every other feldspar variety.

Collector and Market Considerations

For collectors and buyers, the practical guidance is straightforward. Any red or red-orange feldspar described as andesine, andesine-labradorite, or Tibetan andesine should be accompanied by a laboratory report from a recognised facility — GIA, AGTA Gemological Testing Center, Gübelin Gem Lab, SSEF, or an equivalent — explicitly addressing the question of copper diffusion treatment. Without such documentation, the buyer has no reliable means of determining whether the colour is natural or induced.

Naturally coloured red feldspar from Oregon, sold as Oregon sunstone with appropriate laboratory documentation, represents the legitimate and well-supported alternative for buyers seeking red feldspar. Its colour, while sometimes less saturated than the most vivid treated andesine, is genuine, its provenance is verifiable, and its place in the gem trade is uncontroversial.

The broader lesson of the andesine episode — that novelty, attractive colour, and compelling provenance narratives are not substitutes for independent scientific verification — is one that the gem trade has had occasion to relearn more than once, and that remains as relevant today as it was when the controversy first broke.

Further Reading

• GIA Gems & Gemology: Andesine-Labradorite Feldspar (Fall 2009)
• AGTA Gemological Testing Center
• GIA Research: Copper-Bearing Feldspar