Sapphire is one of the three most coveted colored gemstones in the world. It holds its place alongside emerald and ruby; having said that, it’s the “blood” relative of the latter. Both minerals are varieties of corundum and differ only in color, which is subject to the chromophores — chemical impurity elements. In the case of the ruby, there is chromium, whereas in sapphire, we have a combination of iron and titanium. An analysis of the quantitative content of the chromophore in the mineral is a complex process. It is possible only in a well-equipped gemological laboratory.
Corundum deposits are located close to the earth’s surface. They can be found between one to several meters deep, as well as in the mouths of rivers. The conditions of production are the most primitive. If the mine is owned by a company, there can be an excavator, though more often than not workers only get water, a shovel and a sieve. Firstly they look for a place, digging a small area of three to four square meters. If there is solid evidence of corundum’s presence, they will study it more carefully. It can be taken by an excavator or shoveled, placed in a sieve, and then washed.
In Asia, mainly in Burma, there are two methods of extraction. The first one is the official development of the field with ground rental and maintenance workers. The second one is the opening of an office in order to buy corundum near its sources. As a rule, in the pursuit of profit, seekers implement quality gemstones while bypassing their employers. They transfer “discoveries” to a trustee. The trustee sells raw materials to dealers. Credit cards and checks are never accepted as cash is only welcome. But the buyer gets enough time to check if the corundum is natural and creates a certificate. The payment then follows. The equipment for stone cutting and polishing in the mining areas, as a rule, is not characterized by high quality, and for this reason it’s better to have this done in specialized western centers.
It is believed that the vast majority of sapphires from the important fields in Sri Lanka, Madagascar, Kenya and Tanzania are geologically related. The fact is that since the end of the Paleozoic era, before the beginning of the Mesozoic era, there was but one supercontinent, known as Pangaea — where all the continents of the world were once linked. A giant river carried the minerals from a single source across the supercontinent. And when Pangaea broke up under the influence of powerful tectonic forces and continental drift (400-200 million years ago) every mainland formed their own deposits. As a result, the stones from one superdeposit (scientists suggest it was the island of Ceylon, known as Sri Lanka) is now at a distance of several thousand kilometers apart.
Within the rankings of the most valuable sapphire deposits, first place no doubt belongs to Kashmir. The source itself was fully developed only seven years after opening in 1880. Thus all Kashmiri sapphires on the market were mined long ago. The step below is corundum from Burma. In contrast to the specimens found in the territory of Sri Lanka and Madagascar, Burmese sapphires have virtually no purple color, which would be highly valued by experts. But the percentage of gem quality in ratio to the entire production volume is much lower than in other countries, therefore, sapphires from Burma have a higher price. Third place goes to Sri Lanka and Madagascar. And after, African deposits, as well as Thailand, Vietnam and Cambodia.
The influence of a sapphire’s color on its value
One of the most important features of sapphire is its color. Moreover, even the most seemingly insignificant difference in shade to the untrained eye may cause a noticeable difference in the price. By itself, corundum is completely colorless. However, it is rare that we would meet it in its pure form in the natural world. Frequently there are coloring trace elements (the aforementioned chromophores) such as iron, titanium and chromium, present in its crystal lattice. The benchmark of the classical sapphire has a color of a velvety-blue to blue, with a barely noticeable purple hue, saturated with medium ormedium-dark tones — it is called the “Royal Blue”. The examples that meet this requirement set the highest price per carat. At the other end of the price scale there are greenish-blue, muddy-swamp, and grayish, overly bright, or, conversely, very dark stones of a commercial grade.
If a piece of corundum falls neither into the category of rubies, nor classical blue sapphires, they are called fancy sapphires. Such stones pass through the wide range of colors and shades. But here we have the same rule of selection: the more intense and uniform the color, the more valuable the mineral itself.
Zoning and color change sapphires
Sometimes, when a seemingly perfect sapphire is illuminated by a flashlight or viewed in front of the sun, it may reveal the growth line (the zoning in the crystal), with lighter and darker areas. When cutting these stones it will cost much less to do those whose color is evenly distributed throughout the whole volume of the gem.
Another “insidious” form of corundum is those that are a rich blue in daylight, while with a lamp shows a bright purple hue. In lab reports such minerals are called “color change sapphires”, and they are cheaper than the classical ones by 50-70% of the price.
Inclusions and «silk effects» in sapphires
In addition to zoning, even the expensive samples of corundum may have inclusions that can be clearly seen when illuminating the crystal from the bottom. Highly transparent stones are extremely rare. As a rule, any imperfections in the stone make its appearance less attractive. Often, Kashmir and Burmese sapphires have tiny inclusions in the form of the finest rutile needles. They give corundum the “silk effect” and make it cloudy. But if rutile presents only a bit, almost as if it is absent, the sapphire gets a very beautiful soft, velvety appearance, whilst at the same time remaining transparent — this makes the stone more valuable.
Heat treatment and diffused sapphire
It is important to know that the majority of sapphires on the market are treated. As a rule, uncut grayish-bluish-green stones are heated — after that their color becomes more saturated, removing a greenish hue and improved cleanliness. In order to detect traces of enhancement one would at least need a magnifier with a tenfold increase in zoom, but more often so, a microscope: in heat treatment, inclusions in the stone are modified, and the specialist will immediately notice this.
For ready-cut stones, in the last few years thermo-diffusion processing has become increasingly used. If it is necessary to obtain a blue corundum, it is heated to about 1800 °C with titanium. This chemical element penetrates into the surface layer of stone and reacts with iron, which is almost always present in the crystal lattice of sapphire. As a result, the mineral surface becomes a saturated blue. Identifying stones with surface diffusion is easy even for the non-gemologist. One can simply place it on a sheet of white paper pad-down, and then highlight it with a flashlight from the bottom — in the center it will be pale, and closer to the girdle it will be darker, forming a “frame”.
There is also another kind of diffusion with beryllium, which is used for yellow and orange stones. It’s important to note that beryllium is present in natural corundum, namely ennobled and recognized because the mineral’s rate exceeds 5 ppm (parts per million). Diffusion sapphires are typically valued as less than stones processed by only heating. Their price ranges from fifty to thousands of dollars per carat, depending on the size and quality of the cut.
The average yield of the raw material is from 30 to 50%. And only the cutter decides whether he will receive a 10-carat poor quality sapphire, or a 6-carat decent one from the 20-carat uncut material. From a different side, corundum may show a different shade of color. For example, blue stones are greenish-blue or violet-blue. Furthermore the color often shows the crystal growth (the zoning). In such a way, the cutter’s task is to not only eliminate unwanted shades to give the stone a beautiful color, but also to save the maximum possible weight.
But, unfortunately, oftentimes bets are made on carats and lack of information for customers. And most sapphires on the market have poor symmetry. When cutting, their parameters are not kept, and as a result, such stones do not play along and do not look beautiful. As with rubies, sapphires of rectangular shapes are the most expensive. The diamond round cut is rarest, as it dissipates the color. However, oval and cushion ones, which enhance the saturation of stone, on the contrary, prevail.
Sapphire jewelry wearing
Sapphires gain an impressive 9 points on the Mohs scale. But after long, sloppy wearing, the visage can be covered with small scratches. You can correct this acquired defect only by re-polishing the mineral, to which it loses about 0.5% of weight. After such a procedure, the updated sapphire needs a new lab report as it doesn’t comply with the old document anymore.
The demand for the highest dignity sapphires is continuously growing. And the chain of world price records for the Kashmiri blue gem corundum by the carat perfectly shows it.
The last of them is a ring with a stone weighing 27.68 carats, sold at Sotheby’s in October 2015 for $6.7 million, equating to $242 thousand per carat. The previous record belonged to a 10.33 cts sapphire. Said sapphire was put under the hammer at Christie’s in June 2015 for $2.48 million, or $240 thousand per carat. Even earlier, in October 2014 at the same auction, Sotheby’s sold two rings with top sapphires — the first one weighed 12 cts, and was sold at $194 thousand per carat, while the second one, which weighed 17.16 cts, hit $236.5 thousand per carat. A bit before in April 2014, a mineral of 28.18 cts was sold at Sotheby’s for $180 thousand. Moreover, in the case of a pair of earrings, in which the stones are optimally matched in size, color and clarity, their value increases by at least 30%.
Unlike diamonds, colored gemstones don’t have any well-structured evaluation system that would help to determine their real value. There are hundreds of different shades of blue. And it is difficult to bring them under some general classification with letters or numbers. A sapphire’s value can range from thousands to hundreds of thousands of dollars per carat. It depends on its color, clarity, origin, weight, and the presence or absence of treatment traces. Thus, there is no need to burn a hole in your pocket by spending silly money. When buying corundum you just need to consult a specialist. Regarding labs, only three companies in the world can give a comprehensive assessment of sapphires — Swiss Gübelin, SSEF and the GRS. But even the existence of such a report doesn’t help a non-gemologist to determine the real value of the stone. The decision is always yours.