Untreated vs. Treated Gemstones: What's the Difference – and Why Is It So Important?

The most important distinction in the gemstone market – and why it determines value, price, and trustworthiness.

Imagine buying a painting – without knowing if it's an original or a retouched reproduction. Many gemstone buyers find themselves in precisely this situation without realizing it. This is because the market for colored gemstones is full of stones that have been optically enhanced through heat, oil, irradiation, or chemical agents – often without any disclosure.

Yet, the distinction between untreated and treated stones is one of the most fundamental aspects of gemstone purchasing – it fundamentally influences value, price, and long-term appreciation.

1. What does "treated" mean for a gemstone?

Treatment refers to any human intervention that alters a stone's natural properties after its formation. The most common methods are:

•       Heating: By far the most common treatment. Sapphires and rubies are heated to up to 1,800 °C to improve color and dissolve inclusions. Over 90% of all sapphires on the market are heated.

•       Fracture Filling: Standard practice for emeralds. Cedarwood oil or resin fills natural fractures, making the stone clearer. The degree of filling – from minimal to heavy – is crucial for the value. 

•       Irradiation: Mainly used for topaz, tourmaline, and some sapphires to intensify or change color. Some irradiation effects are permanent, others are not.

•       Diffusion Treatment: Chemical elements are introduced into the stone's surface at high temperatures to create color. Especially known for sapphires. This treatment is not stable and can disappear during repolishing.

2. What does untreated mean – and why is it so rare?

An untreated gemstone is exactly that: a natural product that has not undergone any human intervention since its formation millions of years ago. It has only been cut and polished – everything else is purely the work of nature.

This is exceptionally rare. A sapphire that naturally has a beautiful, saturated color without ever having been heated – that is geological chance at its highest level. Accordingly, the demand among collectors, investors, and discerning buyers is enormous.

3. What is the practical price difference?

The difference is significant – and grows with the quality of the stone. As a rule of thumb:

•       A good quality untreated sapphire fetches two to three times as much on the market as a comparable heated stone.

•       For Kashmir or Burmese origin, the factor can be significantly higher.

•       For emeralds: "No oil" or "minor oil" is investment-grade – anything above that is a mere jewel, not a store of value.

This price difference is also reflected in major auctions: At Sotheby's and Christie's, the treatment status is always explicitly stated in the catalog – and "no heat" is a primary selling point.

4. How do I, as a buyer, know if a stone is treated?

In most cases, it is not possible with the naked eye. Even experienced gemologists need a lab and a microscope. The only reliable method for a buyer: a certificate from a reputable independent laboratory.

Reputable gemological institutes analyze every stone for traces of treatment and document the result in the report. The report is thus not only a document of authenticity but also one of the most important quality features when purchasing.

No certificate – no purchase. This applies particularly to stones valued at €10,000 and above.

Conclusion: Treated is not bad – but it must be declared

Treated gemstones are not fakes – they are genuine natural stones that have been optically enhanced. For jewelry to be worn and enjoyed, they are often a good choice. However, anyone investing in gemstones or wishing to buy a lasting heirloom should opt for untreated stones – certified, documented, and verifiable. Transparency makes the difference.