What is sapphire? [Sapphire Introduction Course 2]

Sapphire is a blue variety of the mineral corundum (Al2O3). Pure corundum substances that are completely free of other trace elements are colorless, while the blue color of sapphire is due to the interaction of trace amounts of iron (Fe2) and titanium (Ti4) through a phenomenon called intervalent charge transfer (IVCT). In sapphire, iron can exist in the form of divalent iron Fe2 or trivalent iron Fe3. These "foreign" trace elements replace the position of aluminum (Al) during the growth of the crystal.

An important gemmological characteristic of sapphires is their polychromatism. Polychromatism refers to the observation of sapphires from different directions. Gems will show two slightly different blue colors. This characteristic is best observed through a polarizer.

Left: two common polychromatic colors in sapphires observed through polarizing filters: blue and purple blue. Right: visible polychromatic colors in cut sapphires. Photo: Michael Krzemnicki, SSEF

Sapphires share many of the same properties as rubies because both are corundum-type gems. Source. Gem-A.

Sapphire: from raw stone to cutting and grinding

According to the appearance of the raw stone and the wishes of the cutting artisan, the raw sapphire stone will be cut and ground into different shapes and cuts. The gemstone cutting artisan hopes to show the best color of this sapphire through cutting and grinding. At the same time, it also reduces the material loss caused by cutting and grinding as much as possible.

Sapphire raw stone sorting in Mogok, Myanmar. Photo: Laurent Cartier, SSEF

The following figure shows different common cutting examples of blue sapphires. Of course, these cutting methods are also applicable to sapphires of other color varieties.

Fine sapphires with different cuts. Photo: SSEF

Unique sapphire varieties: color-changing sapphire and starlight sapphire

The discoloration effect is an optical phenomenon in gemstones. The color of a gemstone with this effect changes according to the light source it touches ("cold" daylight and "warm" incandescent lamps). One of the most famous gems is the gemstone Alexandrite.

As shown in the sapphire in the figure below, color-changing sapphires will show color changes from blue to purple depending on the light source. In natural color-changing sapphires, this color-changing effect is not only iron and titanium (the cause of blue), but also because the crystal contains trace chromium.

This beautiful color-changing sapphire shows a moderate color change, blue in daylight (left) and purple in incandescent light (right). Photo: SSEF.

There is also a well-known "starlight effect" in sapphires. This is a reflection effect caused by light shining on three groups of tiny fibrous inclusions arranged in parallel at 120-degree angles inside the gemstone. This effect is only possible when the gemstone containing the inclusions at these specific angles is in the right direction and is cut and ground into a plain arc (that is, a polished convex arc rather than a facet).

An oval-shaped sapphire with a plain arc surface, when the light shines on it, it presents a beautiful starlight effect. The picture on the right shows the direction of this gem relative to its original crystal and its optical axis (black line). Photo: SSEF. Illustration: Michael Krzemnicki, SSEF

Sapphire: Geology and Genesis

Sapphire is mainly formed in geological environments where silicon is poor, and silicon is one of the most important rock-forming components. Therefore, the host rock that produces sapphire needs to contain a very rich amount of aluminum, which provides the two elements of aluminum and oxygen needed for corundum mineral mineralization. Pure corundum is colorless. To form a colored sapphire, other trace elements are needed in the environment (for example, iron and titanium that produce blue, or chromium, magnesium and vanadium that produce other colors).

The raw ore of sapphire can occur in a variety of rock types, and sapphire can be found in metamorphic rocks and magmatic rocks. Magma is formed by lava (magma) from the depths of the earth rising to the surface and solidifying after cooling. Magma is either produced on the surface of the earth due to volcanic activity or from the depths of the earth's crust (such as granite intrusions). Metamorphic rocks are made of original rocks, whether sedimentary rocks, magmatic rocks, or even existing metamorphic rocks. Due to the increase of pressure and temperature in the earth's crust (for example, during continental collisions and orogenic processes), these original rocks are transformed into a new type of rock during the geological process called metamorphism. Metamorphic rocks; a typical example of such rocks is limestone (sedimentary rocks), which forms Dali through metamorphism.

Except for sapphires in metamorphic rocks (such as skarn, gneiss, and mica schist) and magmatic rocks (such as syenite and alkaline basalt), a large number of gem-grade sapphires are not found in primary rocks. More sapphires are found in secondary sediments formed by rock erosion and weathering, such as gem-bearing placers in river sediments in Sri Lanka.

In the history of the Earth, there are three important periods for the formation of sapphires. The first two periods are related to orogenic events, while the last one is due to magmatic activity, bringing the sapphires in basalt to the surface of the Earth:

First Period: Pan-African Orogenic Period (a few years ago, a year ago), including sapphires from Kenya, Madagascar, Sri Lanka, and Tanzania.

Second Epoch: Cenozoic Himalayan orogeny (the Fourth Epoch), including sapphires from Afghanistan, Myanmar, Kashmir (India) and northern Vietnam.

The third period: the Cenozoic alkaline basalt eruption period (the Quaternary), which includes sapphires from Australia, Cambodia, and Thailand. Such sapphires are formed in the upper mantle or deep in the earth's crust and transported to the surface with the eruption of basalt magma, so they are found to occur in basalt.

When conditions permit, we can use the most advanced scientific methods to determine the formation age of the sapphire by analyzing the inclusions located on the surface of the sapphire. This not only provides evidence for the formation and age of the sapphire, but also can provide support for determining the origin of the sapphire.

In a 20-meter-deep mine that mainly produces sapphires and spinels in Taung-Tut, in the Mogok Stone Track of the Mogok gem belt, miners are digging sapphires in strongly deformed primary rock. Photo: Laurent Cartier, SSEF It's just that It's just that

Color Title: Royal Blue

This Royal Blue 35.09-carat Kashmir sapphire set on the ring. At Christie's Geneva Magnificent Jewelry Auction in 2015, this royal sapphire was sold for US $2 million. SSEF Appraisal Certificate No. 77675. Picture: Christie's Auction House

The more appropriate description of Royal Blue is that it is a saturated blue, pure in color or with a very slight purple hue. Although 'Royal Blue' was historically a unique name for high-quality sapphires only produced in the Mo Valley area of Myanmar, today, other sapphires produced from metamorphic rock deposits, such as sapphires found in Madagascar and Sri Lanka, can also achieve the quality of 'Royal Blue' and obtain this honorable title.

Any type of treatment (such as heating, fissure filling, etc.) will disqualify the sapphire from using the color called. In addition, the sapphire must be relatively free of visible as well as dark-colored inclusions, and the gemstone itself must display a uniform color distribution and vivid internal reflection effect.

A royal blue natural sapphire colorimetric stone used in SSEF to compare colors. Please note that the color of the photo on the screen or on the print may be different from the color of the gemstone itself. Photo: SSEF

Article source: SSEF