Sapphire Heat Treatment

Nature can be a beautiful but messy process. And the world of corundum gemstones is a testament to this fact. Natural sapphires, unlike the synthetic variants, are sourced from the natural world, which means they come with the admixture of a variety of elements, and it is the presence of these various external elements, as we shall see in detail, is that gives sapphires their wonderful colours. However, the presence of these various elements and the colours they give rise to are not perfect. Often times the colour in a rough, uncut gemstone that is just sourced from the natural environment can be worlds away from what one finds in the jewellery store, or what one would wish for in a fine sapphire engagement ring. This is where the process of sapphire treatment comes in. Of course in the world of sapphires, there are corundum stones that are found in an optimal condition that makes them a perfect fit for making fine jewellery, like engagement rings. However, these occurrences are very rare in nature, especially in relation to other factors that determine gemstone quality like its carat, clarity and later on during the process of gemstone cutting, the cut of the gemstone. A reality that opens the door to sapphire treatment.

Trace Elements and the Presence of Color in Sapphires  

When it comes to understanding the need for and the methods that are used for sapphire treatment, it is important to first understand the mineral character of sapphires. This takes us to the study of trace elements, and the atomic structure of the corundum crystal. Sapphires in their pure, natural state are colourlessness stones. The mineral character of corundum, known as crystalline aluminium oxide Al2O3 and how interacts with elements from the external world is important to understanding the presence of colour in gemstones. Which requires a basic grasp of the atomic character of corundum crystals, and their interaction with trace elements like iron (Fe) and Titanium (Ti). The white colour of aluminium oxide requires the presence of trace elements, that enter into it in the course of the gemstone’s natural development. The entry of external elements occurs due to structural imperfections in the conundrum crystal. In this process, the aluminium ions in the crystals are replaced by ions of trace elements like iron and titanium through a process known as intervalence charge transfer. Alright, we are now getting into some physics of crystals: intervalence charge transfer involves the interaction between two metal ions that occurs via the transfer of electrons. In charge transfer, there is an exchange that occurs, where the electrons between two adjacent transition metal ions such as aluminium and titanium or iron. The process of these ion transfers is made possible by the particular crystal structure of the corundum crystal, which we shall see in the next section.

How Blue Sapphires Get their Color 

In corundum crystals like sapphires, the presence of these trace elements at various levels: with respect to the number of ions, causes changes at the atomic level of the gemstone. The changes caused by the presence of trace elements modify the ability of corundum crystal, affecting how it interacts with light. Corundum gemstones in their pure state are not able to absorb the colours in the visible spectrum. The manner in which sapphires interact with light results in the different hues they carry; which in turn points to differences in the underlying elemental character of the gemstone. When it comes to blue sapphires, which is arguably the most popular and most recognized version of the corundum gemstone, pairs of iron and titanium ions (Fe+2 and Ti+4) are substituted for aluminium ions (Al+3). The substitution of these trace element ions into sapphires are made possible owing to the octahedral character of the sapphire, which exist as irregularities, that in turn enable the transfer of ions from other trace elements. In the case of blue sapphires, the presence of the two elements iron and titanium, even in infinitesimal amounts, function as ‘colour centres’ in the stone, allowing the absorption of all colours in the spectrum of light, except, blue, which gives blue sapphires their distinct hue. As noted the presence of these different trace elements in small amounts do not change the character of the sapphire, as sapphires with different hues or the colourless white variety remain fundamentally identical to one another. This now brings us to the subject of sapphire treatment and the kind of effects it has on the gemstone.

Why Sapphires are Treated?

First off what is meant by the word ‘treatment’? In practice, sapphire treatment could mean a number of different practices which are done to change the character of the gemstone, at varying degrees. In this context, we are only concerned with the heat treatment of sapphires, which as we shall see falls into the range of acceptable sapphire treatment methods. In the gemstone industry sapphires are treated (heated) for a variety of reasons. The primary driver being the need to optimize the colour of the gemstone. Since colour is a key factor when it comes to measuring a gemstone’s worth, especially in relation to coloured gemstones like sapphires, there is a greater industry incentive to heat treat sapphire stones. In addition to the heat treatment of sapphires and its effect on colour, the heating of sapphire stones has a way to enhance the gemstone’s other qualities, as the heating of the gemstone is also associated with the reduction of visible imperfections known as inclusions. So when one speaks of heat treatment, what takes place is the enhancement or reinforcement of the gemstone’s inherent qualities. One of the main reasons for treating gemstones is to increase/improve the colour of the gemstone. Since most corundum crystals that are sourced from natures are not ready to meet the high standards of fine jewellery, natural sapphires, as rough stones are treated to intensify their colours, to increase clarity by reducing impurities and structural weakness or to even alter the tone or potentially even the hue of the gemstone.

What Takes Place when Sapphires Stones are Heat Treated?

The heating of the gemstone under controlled conditions will vary from time and place, reasons for heating, and in turn the kind of changes it produces to the underlying character of the corundum crystal. Sapphires as an allochromatic mineral: which means the colour in the gemstone is a function of the light-absorbing character of the crystal-which is a product of structural imperfections that correspond to its atomic structure-one that leaves the door open to external elements, whose interaction, via charge transfer involving the atoms in corundum crystal gives rise to its colour. For the reader’s consideration, much of the research on this still remains experimental, but the dominant one is the molecular orbital theory. When it comes to the heating of gemstones, the presence of trace elements like titanium or iron is a prerequisite. In addition to helping to improve the clarity of the corundum crystal key changes take place to the chromophores (colour causing) elements (e.g. Ti and Fe), such making them become more strongly part of the corundum crystal. During the heating of sapphires, the conditions are changed to impel the iron (Fe) and titanium (Ti) ions (in blue sapphires) to acquire charges ranging from ⁺² or ⁺³. Which affect the final colour of the gemstone making it darker or lighter. In addition to changing the colour tone of the gemstone, which can in practice make them darker or lighter, heat treatment also has the effect of annealing or healing various inclusions found within crystal structures of the gemstone, thus improving the clarity of the treated stone.

Main Factors in the Heat Treatment of Sapphires

To get a bit into the industrial process that involves sapphire treatment. When companies and stakeholders who are engaged in the business of sapphire treatment proceed to carry out this process, there are certain factors that are taken into consideration. They are:

• Time

• Temperatures

• Atmosphere

• External Elements

• Starting Materials

Time (or duration of the treatment) is a key factor. Depending on how long or short the heating takes place, can have a drastically different impact on the final state of the gemstone. The temperatures at which the heating is conducted, also play a part, which we shall see below. The atmosphere or the kind of controlled environment under which the heating takes place. For sapphire heating in the industry takes place under various conditions, depending on the size, scale of the business, and the kind of geographic region in which it is operating it. All of which have an impact on the kind of technologies that are at its disposal. External elements or the use of any kind of chemicals or enhancers that are used however that may vary in the case of expansive diffusion processes, that seek to fundamentally alter the character of the gemstone. The starting materials, which are usually only the corundum crystals used, in the case of sapphire treatments. However, the nature of starting materials is important when dealing with synthetic sapphire variants. As these types of man-made gemstones have entered the mix, it is important to distinguish them from natural sapphires sourced from nature.

The Ethics and Goals of Sapphire Heat Treatment

At this point, it is important that we address some concerns people may raise when it comes to the treatment of sapphires. Whilst the confusions over type, character and extent of gemstone treatments vary, when it comes to heat treatment, the goal is to enhance the extant qualities of the stone, to make it higher end in terms of its market value, via the enhancement of the stone’s clarity and colour. The problems with sapphire treatment, when considering the type of methods that go beyond the basic heat treatment of sapphires emerges when the treatment of the gemstone does not only involve the fixed chemical character of the corundum. For when new chemicals additions are made, or when new trace elements are artificially introduced, fundamental changes occur to the character of the corundum crystal. This is a subject that will be explored in greater depth, for now, the reader must understand that heat treatment of sapphires is an industry-accepted practice that seeks to enhance the qualities of the gemstone, and to make gem-quality gemstones like the wonderful blue sapphire accessible to those who wish to seek it. Unlike certain diffusion processes that seek to alter the character of the stone at a more fundamental, sapphire heat treatment, like any industrial practice follows the approach of extractive industries. Where resources that are taken from nature invariably undergo a process of improvement and change to make them fit for customers, sapphire heat treatment follows the principle.