Ruby

Ruby: The Chromatic Quintessence of Corundum Ruby is the prestigious red variety of the mineral species Corundum, an aluminum oxide with the chemical formula Al₂O₃. It shares the same crystal structure as sapphire but is distinguished by a specific trace element chemistry. With an exceptional Mohs hardness of 9, ruby is the hardest gemstone after diamond, offering unparalleled resistance to scratches and daily wear.

The Chemistry of Red: Chromium and Fluorescence The color of ruby is due to the substitution of some aluminum ions by chromium ions (Cr³⁺).

• Fluorescence Phenomenon: Unlike iron, which "quenches" light, chromium often causes intense red fluorescence when exposed to ultraviolet rays (even those in daylight). This gives certain rubies, particularly those from Myanmar (Burma), a "glowing coal" appearance that seems to shine from within.

• "Pigeon's Blood": In a laboratory context, this term refers to a vivid, saturated red with strong fluorescence and minimal darkening inclusions. It is the gold standard for investment-grade rubies.

Inclusions and Optical Phenomena Inclusions in ruby are essential for proving natural origin.

• "Silk" and Needles: Fine needles of rutile (TiO₂) are often present. If these needles are dense enough and oriented according to the crystal's hexagonal symmetry, they create asterism. When cut into a cabochon, the stone reveals a six-rayed star: the star ruby.

• Dichroism: Ruby is a dichroic gemstone. Depending on the viewing angle, it can shift from purplish-red to orange-red. The lapidary must therefore orient the stone's table perpendicular to the optical axis to achieve the purest possible red.

Laboratory Analysis and Treatments The majority of rubies on the market undergo thermal treatment (heating) to dissolve rutile inclusions and intensify color.

• Expertise: In the laboratory, we distinguish between "naturally" heated rubies (without additives) and rubies treated with lead-glass filling, a practice that significantly depreciates the gem's value. Standard care involves lukewarm soapy water, but glass-filled rubies are extremely sensitive to acidic products and heat.