How to Identify an Authentic Alexandrite Gemstone

Alexandrites have established a reputation as the top colour change gemstone in the world. Notable for the clear contrast of hues that become visible depending on the kind of light the gemstone is exposed to at a given time, or a combination of colours, sets this gemstone apart. It is important to note that alexandrites are not the only gemstones in the world that feature colour change. Other popular gemstones like tourmalines and certain sapphire varieties also feature variants of colour in a single stone. However, alexandrites differ with respect to the colour change effect in that the shift is more definitive. And there are good reasons why the colour change effect in alexandrites differs from other types of colour effects found in various gemstones. In this post we will outline the key aspects of colour change in alexandrites, look at some of the science that underlies this effect, and in the process, we will help you learn what you need to know when it comes to picking a quality gemstone.

What Contributes to Color Change Effect in Alexandrite

To understand gemstone colour, the first point to consider is that almost all major gemstones in the world derive their colour from the presence of trace elements that are present within the gemstone. Trace elements enter the gem during its process of developing deep within the Earth’s surface over the course of millions of years. In alexandrite gems, which is a variant in the chrysoberyl family of minerals, a number of key trace elements combine to contribute to its colour, with the type of trace element combination playing a key part in the quality of colour change the stone is able to feature. As chrysoberyl in its natural state is often found in a colourless form or in a dim shade of yellow, the difference with alexandrites is due to the presence of trace elements that are found in various levels; and they are: iron, titanium and significantly chromium, and in certain instances vanadium being the mainstay.

The rarity of the trace element of chromium in the chrysoberyl, which mainly consists of beryllium and aluminium oxide, is what sets alexandrites apart. Chromium is found in greater abundance in other gemstone types like corundum based crystals like sapphires, and interestingly sapphires are also known to feature this colour change effect. However, the effects tend to vary from weaker to moderate shifts in hue, with stark colour changes that contrasts between major hues that are found in top alexandrites, not being so prevalent or captivating. The presence of the trace element chromium (or in some cases vanadium) is what makes the difference in alexandrite gems. The rarity of these gems is determined by the combination of chromium and beryllium in a single stone, which is very nature is rare, and the absence of silica, which is highly common in nature, sets alexandrites apart from other hardened crystals like emeralds.

The Science of Color Change in Alexandrite Gemstones

If trace elements contribute to the presence of colour in alexandrites, as it is in other gemstones, the question is how does the colour change effect emerge? Alexandrites are defined by their ability to showcase a different colour depending on the type of lighting conditions under which they are viewed: with a bluish-green being the dominant colour in daylight, to a purplish raspberry red light under incandescent light. But how does this happen; what is the mechanism that gives rise to the colour change effect in alexandrite gemstones? The discussion about how colour is perceived is a highly technical subject which goes beyond the scope of this post. What the reader needs to know is that colour is a function of the different wavelengths that make up light and their interactions with different forms of matter, before they reach our vision.   

The Role of Light and Visible Colour Spectrum 

The first thing to consider is that colour is an effect that results from the interaction between light and how it interacts with various elements in nature. The light that is actually visible to us falls within a certain spectrum: with the human eye only being able to detect the range of the light spectrum that falls between 400 nanometers (which is close to purple) to 700 nanometers (which is close to crimson or red). When it comes to alexandrite gems the transmission falls close to the 490 nanometers range: which is the blue-green spectrum of colour, and a second light absorption level, which is around 600 nanometers; which is the closest to the red region. This spectrum is primarily dependent on the presence of the chromium ions, which occupy the lattices of alexandrite crystal.

What sets alexandrite gems apart is their ability to absorb different wave components depending on the type of light the gem is subjected to. In daylight, during which blue short-wave components are strong, it produces blue-green results. An effect which is strengthened by the higher sensitivity of the human eye to green light. In incandescent light, however, long-wave spectral components are strong, thus the human eye is able to perceive the colour red more strongly; which is a function of the low absorption of red in the spectrum. In other words, the colour that is visible in the stone is a result of a combination of factors: the physics of light, the capabilities of the human eye, and the role and character of materials in the natural world.

How to Pick a Quality Color Change Alexandrite Gemstone

When it comes to picking a quality alexandrite gemstone, a number of factors enter the equation. The purpose of the buyer is a key point. Are you a gem enthusiast or collector who is looking to land a rarer coloured gemstone or are you seeking an alexandrite to place on your alexandrite engagement ring? It is important to consider that the colour change effect in alexandrite stones is not uniform, with certain alexandrite stones featuring the colour change effect more strongly than others. Gemstones in this group are sourced from certain parts of the world, featuring a light brown tone, which affects the quality of the colour change of the stone.

The Stronger the Colour Change the Higher the Gem Quality and Value

Thus alexandrite stone which can produce a distinct shift from green to red and vice versa being valued higher. When it comes to picking such stones that feature a stronger colour change effect, a higher level of chromium as a percentage is a key feature. Here the country of origin matters. It is well known that whilst Russia, which historically led the market for alexandrite gems, has fallen away over the decades. Today there has been something of an influx of alexandrite gemstones from parts of Africa, with Tanzania being an important one. However gems of this type from this part of the world have a much smaller level of chromium, in contrast to alexandrites from Brazil, which feature a stronger colour, however, their colour change effect is known to be relatively weaker, according to certain findings. But given the high natural colour, the accompanying colour change effect of Brazilian alexandrite is among the best in the business. 

New alexandrite stones have entered the market from Ceylon (Sri Lanka), however, stones from this part of the world are known to be slightly weaker in terms of colour, owing to the presence of a slightly yellowish to brown shade. However, Ceylon alexandrite does feature solid cuts, and produces a better colour change effect, despite their light to brown to yellowish shade. Further, quality alexandrite stones are able to feature a combination of green and raspberry red hues when exposed to both light types: producing an effect that is almost akin to a disco ball. However, the quality of the colour and glow will greatly depend on the carat and cut of the stone. So finding a jeweller who meets all of these requirements would be key.