Unheated vs Heated Gemstones: What every buyer needs to know

Turning up the Heat

Ruby and sapphire, both varieties of the mineral corundum, are routinely heat-treated to enhance their appearance. This process dissolves silk or tiny needle inclusions, improves colour uniformity by reducing unwanted secondary colours, and intensifies the purity of the primary hue. 

For rubies specifically, heat treatment removes silk inclusions that obstruct the light, improving colour uniformity, and intensifying the purity of its primary hue—red. This process not only boosts the ruby’s appearance but also its commercial value by creating a more consistent, and desirable product for the market.

Heated gemstones are perfectly acceptable, provided that all treatments are clearly disclosed and transparent. A trusted jeweller or gem dealer will always be upfront about any enhancements, so you can feel confident in what you’re buying.

It’s an Art 

Heat treatment is considered both a science and an art form because it requires a deep understanding of the gem’s natural chemistry. It involves carefully controlling temperature, and atmosphere, as well as years of experience to achieve the desired result without damaging the stone.

It’s a high-stakes balancing act which requires calculated precision—too much heat and the stone is damaged, just enough, and its colour comes alive. 

Since every corundum crystal is different, the amount and type of trace elements like chromium, iron, or titanium, the presence of rutile silk, zoning, and inclusions can influence how a stone may respond to heat. Done right, heat can gently coax out a richer colour, dissolve haziness, or sharpen brilliance. Done poorly, it can induce cracks, uneven colours, or permanent damage. That’s why it’s often said that heating corundum is as much intuition as it is science—part chemistry, part craftsmanship. 

The effectiveness of heat treatment depends on the presence of trace elements like iron and titanium in the original material. This process works by manipulating colour centres—atomic-level imperfections or impurities within a gemstone’s crystal structure that selectively absorb light wavelengths, resulting in visible colour. 

Heat treatment can subtly alter the distribution of trace elements within these colour centres. Applying controlled heat in sapphire could modify how iron and titanium interact, reducing overly saturated, dark tones and revealing a lighter, more balanced colour with improved transparency and light dispersion. An excessively dark, inky blue sapphire which may often appear almost black under certain lighting, may be gently lightened through this method. Without these crucial elements present, heating won’t enhance the stone’s colour and may even cause damage.

That said, not all sapphires respond to heating in the same way. The stone’s original chemical makeup is key—if the necessary trace elements aren’t present to form effective colour centres, heating won’t bring out a better colour. If not done correctly, it could lead to cracks, internal stress, or even dull the stone’s natural vibrancy. 

Signs of Heat Treatment in Rubies & Sapphires

Even the most seasoned expert can’t confirm a gemstone is untreated just by looking at it—only a lab can reveal the full truth. Some treatments are easy to spot for trained professionals, such as glass-filled rubies or sapphires with heat-altered inclusions. Other enhancements like low temperature heating or beryllium diffusion are far more subtle and require scientific analysis to detect.

Certified labs and in-house gemmologists can confirm whether a stone is heated or unheated by observing the characteristics of inclusions. Often high-temperature treatments significantly alter these inclusions.

Heated corundum

• Partially dissolved silk inclusions where heat has broken down the needle-like structures.
• “Drippy” fingerprints showing melted or altered healed fractures.
• Discoid fractures are circular, halo-like cracks that form around solid crystal inclusions.
• Disrupted colour zoning where the heating process has altered the natural colour distribution. 

What is Low Heat Treatment?

Low heat treatment is a subtle method used to enhance the colour and clarity of rubies and sapphires, typically at temperatures below 1200°C. This gentle process mimics natural geological conditions without using additives like beryllium or flux. 

Low-temperature heating can sometimes slip through unnoticed, even by experienced hands. The changes are minimal and often extremely difficult for gemmological laboratories to detect. As a result, these stones may be certified as ‘unheated’ or show no definitive signs of treatment, even though low heat has been applied. 

KV Gems follows strict transparency protocols, particularly when handling stones identified by labs as having undergone low heat treatment. “All stones are certified both before and after any recut,” explains Maria Belmont, Director of KV Gems. 

When the stakes are high, a single opinion isn’t always enough. “For higher-value pieces, we typically obtain two to three certificates from leading international labs such as SSEF or AGL. These institutions have proven to be consistent and reliable in detecting even the most subtle treatments,” she says.

According to Belmont, only a few laboratories make a clear distinction between standard heat treatment and low-temperature heating. “In the market, there’s typically no price difference between the two,” she says. “With advancements in technology, we’ve seen cases where pink sapphires previously certified as unheated later tested as heated—something we always disclose to clients. If a buyer wishes to obtain an updated certificate, we fully support that.” 

Identifying Unheated Stones

Unheated rubies and sapphires typically maintain their natural internal features, free from the telltale signs of thermal alteration. Teal and bi-coloured sapphires are particularly striking examples, with their mesmerising interplay of mermaid blues, greens and yellows, and intricate zoning patterns created during natural growth. 

Under magnification, certain key features can reveal whether a corundum is unheated:

Unheated corundum 

• Sharp, intact rutile silk (undisturbed needles).

• Natural, unaltered colour zoning.

• Pristine, angular solid inclusions.

• Lack of discoid fractures or healed features.

For important acquisitions, it’s essential to request a reliable laboratory report from the most respected gem labs, such as Gübelin, SSEF, Bellerophon, AGL or GRS. These institutions have the advanced technology and expertise needed to detect treatments and verify what the eye can’t see. 

KV Gems prioritises sourcing stones that are predominantly loupe clean. Their inventory consists exclusively of gems with exceptional clarity—where any natural inclusions are typically visible only under 10x magnification through a jeweller’s loupe.

Upon selecting a premium unheated corundum, Joe Belmont, Founder of KV Gems advises to look for vibrant colour that’s clean with no zoning. “Pay attention to the shape—ensure the culet is centred with no silk, bubbles, or feathers. Avoid stones that appear too dark, and steer clear of purple tones in blue sapphires and rubies,” he says. He also recommends a practical approach. “Carry a sample colour reference with you when you travel or shop—it helps maintain consistency in your selections.”