Sapphire, the royal blue gemstone with more than one shade of blue.

Natural sapphire crystal.

September, the beginning of the BEST season, autumn. I mean every season has its perks, this summer especially was wonderful with the heat, but fall still is the best. The colour, cool nights, no bugs, scarfs, sweaters, crunching leaves and the smell pumpkin spice coffee in the air. While October is the better of the two, the birthstone for September is better, in my biased opinion. Sapphires, the bluest of the blue gemstones, are more than just blue. Now just to clarify this before we continue, as I stated in the blog about rubies, corundum is naturally colourless, and thus colour comes from impurities within the stone. Sapphire is the blue variety and padparadscha is the pink/orange variety. All other colours are classified as “fancy sapphire”, this is odd as the beryl varieties had unique names but when it comes to corundum in the yellow, violet or green hues then they are referred to as a “yellow sapphire”, “green sapphire” and “violet sapphire”. Confusing, I know, even to a gemmologist. Unless a colour is in front of the word “sapphire” then it is assumed one is referring to the blue variety of corundum. 

The many colours of corundum.
Photographed for the CIBJO project from the Dr. Eduard J. Gubelin Collection.

Sapphires, like some of the precious birthstones, are as old as time itself. Traditionally, the sapphire symbolizes many things, from wisdom, nobility, truth, sincerity, and faithfulness as well as awareness and the “heavens”; blue has always been a symbol of something special to us humans. While there is no real set date we have for when we discovered these stones, the name derives from ancient Hebrew, סַפִּיר (sappir). Some linguists propose that the semitic (e.g. Hebrew) terms derive from Sanskrit Sanipriya (शनिप्रिय), from “sani” (शनि) meaning “Saturn” and “priyah” (प्रिय), dear, i.e. literally “sacred to Saturn”. Other translations from Greek and Latin sapphirus, and Greek σαπφειρος (sappheiros) mean “blue” or “blue stone”.  

Beautiful examples of when colours collide within sapphire. These are some examples of bi-colours sapphires. Montana has a number of yellow, blue and green bi to tri-coloured varieties.

Sapphires originate from many locations. The prized location, which has now unfortunately been mined out and in the midst of a horrible border dispute, is the Kashmire sapphires of the Himalayans, these sapphires are like the designer name brand for sapphires. These sapphires are renowned for their colour intensity and rich blue. Other locations include Australia, Myanmar, Sri Lanka (home of Ceylon Sapphires), Madagascar, Montana in the USA and recent discovery in Nunavut, Canada.   

Rough and cut Kashmire sapphires.

Sapphire is identified as:

  • 1) Trigonal Crystal System, not to be confused with crystal habit. (Note, habit is how the mineral crystallizes)
  • 2) Oxide Composition, 
  • 3) Simple Oxide Family
  • 4) Mineral = Corundum – Al2O3
  • 5) Variety = Sapphire.   

Now for the fun part, how is sapphire blue? This is an interesting question as sapphires change the game a little when it comes to colouring. Unlike ruby where it gets its colour from traces of chromium within the crystal structure, sapphire blue colour comes from the interaction of iron (Fe) and titanium (Ti) with aluminum in the crystal structure. I emphasize the use of interaction here as the iron and titanium don’t just replace aluminum in the crystal structure like chromium. Fe and Ti actually play with valence electrons in the crystal to cause a “colour trap”  us to see blue. In simple terms, Fe and Ti cause small kinks or traps in the atomic structure of corundum that affects the valence electrons of the surrounding aluminum atoms. Light entering the sapphire excites the valence electrons causing them to jump/pass through the trap. As they do they absorb energy, the same wavelength as yellow light from the photons. The result, we see blue as the yellow and some greens are removed from the visible spectrum. In a weird way, sapphires are blue not because something is there, but because of a void, and without light they would actually appear colourless. 

The many shades of sapphire blue.

Now, let’s get serious about sapphires.  With the high price of sapphires, there are many treatments, synthetics and imitations to watch out for when hunting for sapphires. First, some terminology needs to be explained, mainly the terms “natural”, “synthetic”, “imitation” and “treatment”. A natural gemstone refers to a stone made from the earth and is what most people desire/expect. A synthetic gemstone is an identical copy of a natural, just made by man. This means it has the same chemical formula and crystal structure as its natural counterpart. Ideally, the term “synthetic” should only be used if the man-made stone has a natural counterpart. If there is no counterpart, then this stone is referred to as an “imitation”. Imitation is a stone or material representing another stone. Imitations can be natural or synthetic and confuse many people. Cubic zirconia is one of the most common samples for imitating precious stones. Treatment or a treated stone is a natural stone that has been tampered by humans to improve colour saturation, clarity, and durability. There are many interesting ways we have discovered to help improve the natural beauty of precious stones. 

For sapphires, the most common treatment you will see is heating and or surface diffusion. Heat treatment is as the name suggests. The sapphires are slowly heated up to a temperature of from range 800 °C to 1800 °C in an oxygen-free environment, depending on the stone and desired outcome. These stones are then held at this temperature for a few hours or sometimes a few days. Interestingly if the sapphire is heated in an oxygen environment then the colour can lighten, this helps brighten darker, inky blue sapphires, or eliminate blue tones from purplish rubies. The heat actually reduces the iron in the crystal from Fe2O3 to FeO. This transfer increases the presence of the colour trap I explained above, thus a richer, deeper blue colour. This process even works with the other coloured sapphires, like with yellow sapphires. Now these are general temperatures, exact temperatures, times and gases used are a well-guarded trade secret, take it from me I’ve tried searching for some formulas. 

Heat treated sapphires before and after.

Heat treatment can also help increase the clarity of a sapphire. Iron and titanium are also the same elements needed for rutile, the mineral responsible for asterism within sapphires; these silk-like fibrous minerals are what cause the “star” in star sapphires. Sometimes this works in favour of beautiful stone, other times it doesn’t. Heating can help diffuse the rutile threads, dissolving them into the sapphire. This is the case for Geuda sapphires, milky light sapphires from Sri Lanka.  Interestingly if there is no iron or titanium at all then this treatment renders useless. When that happens, another treatment is available.   

An example of a man at work, traditional heat treatment for sapphires are still precticed today in Sri Lanka.

The other treatment I mentioned is surface diffusion treatment. This treatment is similar to the heat treatment, using temperatures around 1750 °C and coating the sapphire with iron and titanium oxide. These stones are then left in this environment for several days. The idea here is to heat the stone so hot that the surrounding ion and titanium begin to diffuse into the surface of the stone. This treatment just enhances the surface colour, not the internal colour. This process unfortunately causes pitting in the surface of the stone and you need to repolish the sapphire. 

This diffusion-treated blue synthetic sapphire shows the shallow depth
of the treatment, confined to a surface layer. Bottom: Diffusion treatment is evident in the
colour concentrations along the girdle and facet junctions. From Robert E. Kane et al.,
“The Identification of Blue Diffusion-Treated Sapphires,” Summer 1990 Gems & Gemology;
photos by Shane F. McClure.Surface diffusion sapphires showing how the colour has been enhanced.

Now for the best part, how can one tell if their sapphire was treated, synthetic, imitation or natural blue wonder? Well, the most obvious answer is to get a professional to look at it. Second, like with all gemstones look at the cost. If the price is too good to be true then it probably is. Sometimes traders don’t know their material and this can work in your favour or against you, buyer beware. If you have a loupe or a strong magnifying glass there are a few things you can look for in your sapphire. For heat-treated sapphires, look and see if there is colour banding. This colour banding is a crucial feature of sapphires. You will see hexagonal or straight bands of blue in natural sapphire. The more defined the banding the better, heating cases this banding to diffuse and turn fuzzy. If you see inclusions, look around them as heat will cause the minerals in the inclusion to expand at a different rate than the surrounding sapphire resulting in fractures.

If you have a REALLY good eye, or a microscope, look for fluid inclusions. These inclusions are like small liquid-filled cavities within the crystal. They are a result of crystallization but they can have three phases within them, solid, liquid and gas. The solid is salt, you will see a perfect cube sitting in there, the liquid is water and the gas is carbon dioxide. The reason these inclusions are important to gemmologists is they are a definitive sign that not only is your stone natural, but that is also was never treated. To identify the diffusion treatment the easiest way to tell is to immerse the stone in water. Since the treatment is the only surface deep and you have to repolish the facets of the stone, when a sapphire that has been treated this way is immersed in water you will see colour concentrating along the facet edge. It looks like a “web” of blue    

This The photo on the left shows (left to right) a diffusion-treated synthetic sapphire from JTV, a diffusion-treated synthetic sapphire from Gem Resources, a flame-fusion synthetic sapphire from Gem Resources, and a Czochralski synthetic sapphire from Union Carbide Corporation. Right: Viewed in immersion, diffusion-treated blue synthetic sapphires clearly show outlined girdles and facet junctions (top), unlike the as-grown blue stones (bottom). Photos by Jennifer Stone-Sundberg.

Now, what about natural vs synthetic Sapphires. A flawless gem is the most prized gem but also one of the most difficult to identify as inclusions help greatly. When it comes to synthetic stones, you’ll need a microscope or maybe even an X-ray microscope to discern the origin of your sapphire. All gemstones leave clues as to how they formed within the crystal structure and carry trace element signatures. While the chemical signature is the best way to identify a stone’s origin you have to destroy a piece of your stone. I don’t think many people would be interested in having a hole in their great grandmas sapphire to see if it’s natural or synthetic. Thus by using high powered microscopes and polarized light, gemmologists can see signatures as to how gemstones crystallized and your stone is left untouched. Synthetic sapphires are produced the same was as rubies, through the flux-melt method or flame fusion method. These methods should be disclosed when selling synthetic/lab-created sapphires and if not, ask. Since I already told you about flame-fusion corundum, this time I will explain the flux-melt method.   

Colour banding within sapphire. To the left, we have parallel and angled banding in a natural sapphire. To the right, we have the curved striae within a synthetic sapphire created from the flame fusion process.

The most common synthetic sapphire is the flame-fusion sapphire, but there are some produced via the flux-melt method. Identifying them is the same so if you wanna brush up on some gemmology for identifying flame-fusion sapphires, ready July’s blog. Upon revising my notes for this blog I realized I should have discussed the flux-melt method for July as this is the method of choice for rubies. However, the flux-melt method can be used for multiple gemstones. In short, for the flux-melt, you take all the ingredients you need for a gemstone, add a flux to lower the melting temperature (lithium molybdate and lithium vanadate for example), put the mixture in a kiln and bake. It’s that simple, but the actual method and ingredients are a guarded trade secret and can take sometimes WEEKS to produce a gemstone. To identify these stones, look inside. Most flux-melt gemstones will have remains of the flux within the crystal. These will look like liquid metallic blobs within the sapphire. You will also notice a lack of any other inclusions or colour banding. 

Whisp inclusions present in flux-melt sapphire.
Inclusions present in a natural sapphire. The large clear crystals are the mineral zircon and the shinny silky fibres are rutile needles. These are the needles responsible for the asterism star sapphire.

Now if you want to search for your sapphire, just like all the other precious stones out there, it is a buyer-beware world out there. Ask questions, and use your gut first. Also, look at the name. Many stones have the name “sapphire” in them to make you think they are related. Again if it’s synthetic, the process should be indicated with the purchase. Imitation stones will be easier to spot as they will not be as hard or will not show the pleochroism. Common imitations for sapphire include CZ, glass, synthetic blue spinel, blue topaz, blue zircon and sometimes iolite. Again best this is to ask a gemmologist about your sapphires, they have the knowledge and experience with these stones and can tell you if what you have is natural, synthetic, enhanced or an imitation. 

What about star sapphires? Yes, they exist naturally and, like star rudies, are the result of thin mineral hairs of rutile crystallizes alongside the triangular habit/system of the mineral. When these hairs are cut into a dome or cabochon shape along with the rest of the sapphire, the hairs will reflect light like a spool of thread. With three spools come three reflections thus we see a star. This is a phenomenon known as “asterism”, and if there is only one reflection like a slit, it’s known as chatoyancy, or ‘cat’s eye’. Now with sapphires, the rutile hairs and the blue colour are affected by the same element, titanium. It’s a delicate balance when treating a star sapphire as you may get a blue stone but no star or a beautiful star and no blue.      

An exquisit example of a natural star sapphire. You can see the 6-point star created from the fine rutile needles within.

As we go through the months I hope each month you learn something new about your birthstones. There is ample to read about the stones for both lore, mysticism, history, formation and identification. Wikipedia and GIA are great places to start if your curious or I have some books below for you. 

Gemmology by Peter read – A gemmologist bible, make sure to look for the latest addition as there is always new information to be added to textbooks. This book covers ALL the gemstones as well as how to use the tools to identify naturals, synthetics and imitations. Warning this book is very technical. 

Jewels: a secret history by Victoria Finlay – an excellent read into the history and lore behind some of the more renowned gemstones.  

A natural sapphire crystal and it’s beautiful cut counter part.

Well, that was another novel and a half, I hope you enjoyed this blog about the sapphires. In October I will be informing you about either opal or tourmaline I have yet to make up my mind as I have some interesting stories about both. 

Cheers, Peter