JUNIOR MINING SPECIAL — Heat-treatment enhances sapphires

Unlike diamonds, which need only be cut to improve their value, rubies and sapphires generally require heat treatment to enhance their clarity and color. Nowadays, 95% of all sapphires and rubies produced worldwide are routinely heat-treated for this purpose.

Recent advances in heat treatment have rendered many sapphire deposits economic to mine. A good example of this is in the northwestern U.S., where American Gem (TSE) hopes to use the process to upgrade the quality of alluvial stones on its 30,000-acre holding near Helena, Mont. Historically, several million carats of sapphire have been produced in Montana since the late 1800s. Unfortunately, most of the stones were not gem-quality without further treatment.

Sapphires, like rubies, consist of aluminum and oxygen in a ratio of 2-to-3. Thus, pure sapphire is Al2O3, also known as corundum. In its pure form, aluminum oxide is colorless. It is the presence of trace elements in the crystal lattice of the natural stone that gives these gems their color. If the only impurity is red iron oxide (Fe2O3), the sapphire will be pale yellow. If chromium oxide (Cr2O3) is the only compound present, the sapphire will be pink to red.

By convention, a pink sapphire is called just that, “pink sapphire,” while a red sapphire is called a “ruby.” If both iron oxide and chromium oxide are present, the sapphire is orange, and such stones are often referred to as “padparadschas.”

The process by which green and blue sapphires are created is slightly more complex. In this case, both iron oxide (FeO) and titanium dioxide (TiO2) are required. If these exist next to each other in the crystal lattice, then a blue color results. If there is much more iron than titanium, the prevailing color is green.

The creation in nature of a clear, colorful sapphire or ruby requires that it be formed under near-perfect geological conditions. In fact, most sapphires and rubies are formed under less-than-perfect conditions and, as a result, the trace elements that would ordinarily color a stone precipitate out as microcrystalline impurities known as “silk.” These impurities give the stones a dull, cloudy appearance.

Fe2O3 and Cr2O3 are soluble in sapphires and in nature they are invariably dissolved in the crystal. FeO and TiO2, on the other hand, have low solubilities and often occur as silk (needle-like crystals). Thus the object of heat treatment is to dissolve the precipitated material within the crystal to improve clarity. The treatment is also used to induce solid-state chemical reactions among the dissolved impurities, thereby enhancing the color.

To achieve these goals, the stones are heated to extremely high temperatures (as high as 1,850 C) in a controlled gas environment.