Understanding Gemstone Treatments: What Every Collector and Connoisseur Should Know

By Lisa Millwood – Expert Gemmologist & Registered Jewellery Valuer

When it comes to coloured gemstones, treatments are a widespread and accepted part of the industry. However, for buyers, collectors, and even jewellers, understanding what treatments have been applied, how they affect value, and how they can be identified is crucial to appreciating — and appraising — a gemstone correctly.

As a gemmologist and registered jewellery valuer in Australia, I frequently encounter treated stones in both antique and contemporary jewellery. This post will shed light on the types of treatments, their prevalence, how they’re detected, and when they impact value.

What Is a Treated Gemstone?

A treated gemstone is one that has undergone a human-induced process to improve or alter its appearance — typically to enhance colour, clarity, or stability. These treatments range from traditional and accepted to controversial and deceptive, depending on the method, disclosure, and permanence.

How Common Are Gemstone Treatments?

Gemstone treatments are extremely common in the market. In fact:

• Over 90% of sapphires sold globally are heat treated.

• Nearly all emeralds are oiled to improve clarity.

• Many rubies and topaz are routinely treated, and even diamonds can be HPHT treated or fracture-filled.

In the commercial jewellery space, untreated stones of fine quality are the exception, not the rule — and this is reflected in both rarity and value.

Types of Gemstone Treatments

Here are the most frequently encountered treatments in valuation and gem identification:

1. Heat Treatment

• Purpose: Enhances colour and/or clarity.

• Common In: Sapphire, ruby, tanzanite, zircon.

• Detectability: Often detectable under microscope; inclusions may show signs of heat.

• Value Impact: Generally accepted; unheated stones with fine colour fetch premiums.

2. Fracture Filling

• Purpose: Improves apparent clarity by filling surface-reaching fractures with glass or resin.

• Common In: Ruby, diamond.

• Detectability: Readily visible under magnification; flash effect is a key giveaway.

• Value Impact: Significant. Filled stones are worth far less and must be disclosed.

Detailed examination of this ruby shows bubbles and a blue flash, indicative of glass filling and drastically reduces its value.

3. Oiling (and Resin Impregnation)

• Purpose: Improves clarity by masking fractures.

• Common In: Emerald.

• Detectability: Can be seen under magnification; some require advanced instruments.

• Value Impact: Light oiling is common and accepted; heavy or synthetic fillers reduce value.

4. Irradiation

• Purpose: Alters colour.

• Common In: Blue topaz, some diamonds.

• Detectability: Often undetectable without lab equipment.

• Value Impact: Generally lower than natural-coloured equivalents; must be disclosed.

5. Dyeing

• Purpose: Adds or enhances colour.

• Common In: Turquoise, jade, lapis lazuli, agate.

• Detectability: Visible under microscope or with filters; dye concentrations in fractures or surface areas.

• Value Impact: Considerable. Dyed stones are generally of much lower value.

6. Diffusion Treatment

• Purpose: Adds a coloured layer via chemical diffusion during heat treatment.

• Common In: Sapphire (especially blue and padparadscha types).

• Detectability: Requires magnification and possibly spectroscopy; colour concentrations near surface.

• Value Impact: Major. Diffusion-treated stones are much less valuable.

Does Treatment Always Affect Value?

Not always. Some treatments, like traditional heat treatment of sapphire, are widely accepted and factored into market pricing. However, undisclosed or unstable treatments — such as glass-filled ruby or dyed turquoise — can greatly reduce value, especially if misrepresented.

The key is transparency. Treatments must be disclosed at the point of sale, and valuations must account for the nature and extent of the enhancement.

Can Treatments Be Detected?

Detection depends on the treatment and the gem. Many can be identified using:

• 10x loupe and microscope

• Immersion techniques

• UV light and filters

• Spectroscopy and advanced lab testing

As a qualified gemmologist, I use a combination of tools and techniques to identify treatments and include this analysis in all formal valuation reports.

Why This Matters for Valuations

As an Australian registered jewellery valuer, I place a high emphasis on the accurate identification and disclosure of treatments, as they directly affect:

• Insurance replacement value

• Resale or estate planning estimates

• Ethical considerations for retailers

Clients should feel confident that what they own or are purchasing has been assessed with full transparency and expertise.

Final Thoughts

Gemstone treatments are an integral part of the jewellery world — neither inherently good nor bad — but they must be understood. Knowing whether a gem has been treated, and how, empowers both buyers and professionals to make informed decisions.

If you're unsure whether a gemstone you own is treated or want to understand its value in today’s market, I offer independent, science-backed valuations and consultations.

Need help identifying or valuing a treated gem?
Visit www.lisamillwood.com or contact me directly to book an expert appraisal.