A precious stone symbolizing fidelity and purity, sapphire is one of the most prized gems in high-end jewelry. It fascinates with its azure-blue color and, like emeralds and rubies, arouses great curiosity. Where does sapphire come from? How are sapphire stones formed? What explains the variety of its colors?
The origins of sapphire
The use and trade of sapphire dates back to antiquity. The Greek language calls it “Sappheiros”, meaning blue, but its oldest name comes from the Hebrew “’sappir”. Since then, sapphire has been used in lithotherapy by ancient civilizations such as Egypt. From the Middle Ages to the Renaissance, collective thought linked the sapphire stone to the sphere of the divine, assigning it a protective power. Sapphire thus became the prerogative of the Church, monarchs and nobility.
Geographically, most sapphire gems come from mines in Asian countries such as Sri Lanka and India. Myanmar (formerly Burma), Cambodia and Thailand are also home to large sapphire deposits. The precious gem is also extracted from mines in Madagascar, Tanzania and Australia. Sri Lanka and India, however, remain the largest exporters of the precious stone.
Since ancient times, sapphire has been recognized for its magnificent blue color and extreme strength. Its exceptional hardness has earned it a rating of 9 on the Mohs scale. Sapphire corundum should not, however, be confused with lapis lazuli, which is a gem studded with gold dust and pyrite inclusions.
The blue color of sapphire stone is not the only hue available for this corundum. Over the ages, a wide range of sapphires has been discovered all over the world. Pink, yellow, green, violet, black and white stones exist and are used in jewelry-making.
How is sapphire formed?
Sapphire gemstones are created by chemical reactions beneath the earth’s crust. Corundum crystals are most often formed in metamorphic and igneous rocks. Schist, granite and gneiss are the most suitable rocks for sapphire formation. Sapphire also crystallizes in magmatic rocks such as basalt and pegmatite.
The presence of aluminum and silicon in a metamorphic rock is the inescapable condition for corundum formation. When this rock is filled, the passage of magma at a temperature in excess of 2,000° triggers the chemical reactions of fusion. These are the basis of sapphire formation.
Some stones then show fine particles in their pure state. These are due to the presence of titanium dioxide within the crystals. In general, these inclusions represent fine rutile needles that give the sapphire stone its brilliance. In gemmology, the arrangement of these needles is called a fingerprint and helps determine the stone’s origin. The shape of the needles is also at the origin of the relatively rare star sapphire, most often cut as a cabochon.
After formation, the sapphire stone is found in alluvial or eluvial deposits as a result of movement and pressure. The gem is then extracted by simply washing the sediments that contain it.
Why do sapphires come in different colors?
Sapphire corundum was originally a colorless mineral. Its hues are due to the presence of chromophoric transition elements. These replace the aluminum in the parent rock and affect the material’s absorption of light. The sapphire stones formed in this way may be of a single color, or an assemblage of pigments not absorbed by the crystals.
Blue sapphire, the most famous, is formed by the presence of iron and titanium in the rock. The color blue comes in a variety of shades:
- cornflower blue from Kashmir,
- royal blue from Ceylon or Myanmar,
- pastel blue from Sri Lanka,
- midnight blue from Thailand.
Colored gems such as yellow sapphire and green sapphire owe their hue to the presence of magnesium in varying proportions. Pink gems, on the other hand, are highly sought-after for their intensity of color, especially above 5 carats, and are formed by the presence of chromium below 2%.
In contrast, there is no naturally occurring red sapphire. The red hue seen on some jewels set with sapphire corundum is obtained by heating.
The extremely rare Padparasha sapphire (lotus flower in Sinhalese) is the only stone with a natural orange-pink assemblage.
