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How To Tell If Ruby Has Been Heat Treated?

by Madonna

Rubies, with their captivating red hues, have fascinated humans for centuries. These precious stones are often subject to various treatments to enhance their color and clarity. One of the most common treatments is heat treatment, a process that has been practiced for thousands of years. For gem enthusiasts, collectors, and professionals, knowing how to identify a heat-treated ruby is crucial. In this article, we’ll delve into the world of rubies, explain the heat treatment process, and provide practical ways to distinguish between treated and untreated stones.

Understanding Rubies and Their Natural Characteristics

Rubies are a variety of the mineral corundum, which is aluminum oxide (Al2O3) with trace amounts of chromium that give the stone its red color. Naturally occurring rubies often contain inclusions and imperfections. These can range from tiny mineral crystals to internal fractures, which affect the stone’s clarity and, consequently, its value.


The Purpose of Heat Treatment

Heat treatment is employed to improve the color and clarity of rubies. This process involves exposing the stones to high temperatures, often between 800°C and 1,900°C (1,472°F to 3,452°F). The intense heat can dissolve some of the inclusions, reduce silk (rutile inclusions), and enhance the ruby’s overall appearance. While heat treatment is widely accepted and significantly boosts the marketability of rubies, it is essential to disclose this information, as treated rubies are generally less valuable than their untreated counterparts.


The Heat Treatment Process

Initial Inspection and Cleaning

Before heat treatment, rubies are carefully inspected to assess their current state. This includes evaluating their color, inclusions, and any existing fractures. The stones are then cleaned to remove any surface impurities that could interfere with the treatment.



The cleaned rubies are placed in a furnace, where they are gradually heated to the desired temperature. The heating process can vary in duration, from a few hours to several days, depending on the stone’s initial condition and the desired outcome.


After reaching the target temperature, the furnace is slowly cooled down. Rapid cooling can cause thermal shock, leading to cracks and other damage. The gradual cooling process ensures the structural integrity of the rubies is maintained.

Identifying Heat-Treated Rubies

Microscopic Examination

One of the most reliable methods to determine if a ruby has been heat treated is through microscopic examination. Gemologists use high-powered microscopes to look for telltale signs of treatment. Here are some key indicators:

Altered Inclusions: Natural rubies often contain silk inclusions, which are fine needle-like inclusions of rutile. When a ruby is heat treated, these inclusions may partially dissolve, become shorter, or appear disrupted. This alteration is a strong indicator of heat treatment.

Heat Cracks: These are fractures that occur due to the rapid expansion and contraction of the stone during the heating process. They often appear as jagged, irregular lines within the ruby.

Recrystallized Areas: Sometimes, parts of the ruby may appear to have undergone partial recrystallization due to the intense heat. These areas can be identified under magnification by their distinct texture and appearance.

Color Zoning

Color zoning refers to the presence of distinct color bands within a ruby. In natural rubies, color zoning can be uneven and random. However, heat-treated rubies often exhibit more uniform and homogenized color, as the heat helps to even out the color distribution within the stone. Examining the ruby under a microscope or a loupe can reveal these patterns.

UV Fluorescence

Rubies can exhibit fluorescence when exposed to ultraviolet (UV) light. Natural rubies typically show a strong red fluorescence due to the presence of chromium. Heat treatment can alter this fluorescence, either enhancing or diminishing it. Comparing the ruby’s fluorescence to known standards can provide clues about whether it has been heat treated.

Spectroscopic Analysis

Advanced spectroscopic techniques can provide detailed information about a ruby’s chemical composition and internal structure. Instruments like Raman spectrometers and Fourier-transform infrared (FTIR) spectrometers can detect subtle changes in the ruby’s molecular structure that result from heat treatment. These changes can include the breakdown of certain inclusions and the presence of specific trace elements.

See Also: How to Tell If a Ruby Is Real with a Flashlight?

Comparing Natural and Heat-Treated Rubies

Price and Value

Natural, untreated rubies are generally more valuable than their heat-treated counterparts. This is because untreated rubies are rarer and have not undergone any artificial enhancements. When purchasing a ruby, it’s crucial to obtain a certificate from a reputable gemological laboratory that indicates whether the stone has been treated.

Aesthetic Differences

While heat-treated rubies can be visually stunning, boasting intense and uniform colors, some collectors and connoisseurs prefer the natural, often more subtle, beauty of untreated rubies. The choice between treated and untreated rubies often comes down to personal preference and the intended use of the stone.

Ethical Considerations

The disclosure of treatments in gemstones is a critical ethical issue in the jewelry industry. Buyers have the right to know whether a ruby has been heat treated, as this information affects the stone’s value and longevity. Reputable sellers will always disclose any treatments and provide documentation from recognized gemological laboratories.

Practical Tips for Buyers

Ask for Certification

When buying a ruby, always request a certification from a respected gemological laboratory. This certificate should specify whether the ruby has been heat treated and provide details about its origin, carat weight, and other important characteristics.

Buy from Reputable Dealers

Purchasing from reputable jewelers and dealers who have a history of transparent business practices can significantly reduce the risk of buying a misrepresented stone. These dealers are more likely to provide accurate information and proper documentation.

Learn the Basics

Familiarize yourself with basic gemological terms and concepts. Understanding the characteristics of rubies, such as color, clarity, and inclusions, can help you make more informed decisions when purchasing these precious stones.


Identifying whether a ruby has been heat treated requires a combination of visual inspection, microscopic examination, and advanced analytical techniques. While heat treatment is a common and accepted practice in the gem industry, the distinction between treated and untreated rubies is essential for buyers and collectors. By learning to recognize the signs of heat treatment and understanding the implications, you can make more informed choices and appreciate the true value of these magnificent gemstones.


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