What Are Sapphires?
Sapphires, and their close cousins, rubies, are members of the corundum mineral species. In gemology, a “species” is a mineral that has a definite chemical formula and a specific three-dimensional structure.
How Are Sapphires Categorized?
A “variety” is a subgroup of a mineral species. Corundum (a mineral species) comes in a number of different varieties. Many of these varieties are not as rare or valuable as sapphire. “Emery” is a common variety of corundum that is used as a commercial abrasive. Old lawn chairs may be coated with a thin layer of corundum if their aluminum surfaces have oxidized.
Different varieties of corundum are distinguished by characteristics of color, transparency, internal features, and optical phenomena. As a variety of corundum, sapphires come in all colors, except red. In essence, rubies are red sapphires because they belong to the same corundum species, just a different variety.
The Chemical Makeup Of Sapphires
Sapphires and rubies are corundum, which is an aluminum oxide (Al2O3.). Corundum has a regular crystalline structure formed by repeating patterns of arrangement at the atomic level. Crystalline minerals are classified by seven different crystal systems that are separated depending on the symmetry of their repeated atomic units.
Corundum has a trigonal crystal structure and is composed only of aluminum and oxygen. It requires an environment free of silicon in order to grow. As silicon is a very common element in the earth’s crust, natural corundum is relatively rare to find. In it’s purest form, corundum is colorless and clear, resulting in white sapphires. It is only with the additional of trace elements that corundum gains its rainbow of hues.
The blue color in a blue sapphire comes from the mineral titanium that is within the crystal. The higher concentrations of titanium in the sapphire, the more color saturation. Too much color saturation can create a dull or overly dark effect in the blue sapphire, which is not desirable and lowers the price of the stone. Blue sapphires also need trace amounts of the following element, iron.
The presence of iron as a trace element in corundum produces green and yellow sapphires, and when mixed with titanium, produces blue sapphires.
There are two very different causes of yellow coloration in sapphires. The most common cause is the trace element iron. In general, increased iron concentrations will increase color saturation. The trace element titanium causes an undesirable green cast in yellow sapphires, and the most valuable stones are relatively titanium-free.
Yellow sapphires can also be colored naturally by low-level radiation within the earth or by lab-induced irradiation. Lab-induced sapphires are not harmful to humans or radioactive but their color has been known to fade with exposure to heat and light. For this reason, most consumers avoid them.
Besides irradiation, yellow sapphires are frequently subjected to intense heat and diffusion treatments. Heat treatment transforms worthless rough into marketable gems, vastly increasing the supply of attractive yellow sapphires on the market today. Unheated yellow sapphires with strong saturation remain quite rare.
Most pink sapphires are colored by traces of chromium. Very high chromium concentrations will create a ruby, and lower concentrations create pink sapphires. If the trace element titanium is also included in the crystal structure, the sapphire will have a more purplish pink hue. Padparadscha and orange sapphires need the presence of both iron and chromium.
Purple sapphires get their coloring from the presence of the trace mineral of vanadium. The element is named after Vanadis, the old Norse name for the Scandinavian goddess Freyja. Vanadium occurs naturally in about 65 minerals and fossil fuel deposits, and is the 20th most abundant element in the earth’s crust. The purple hue in sapphires forms from a small amount of vanadium; a larger amount will cause the creation of a color change sapphire.
Corundum has some pretty interesting properties aside from the possibility of many beautiful colors, and we discuss that next in Durable, Rare, and Unique Properties.