What’s the difference between rubies and pink sapphires? According to one old joke, that depends on whether you’re the buyer or the seller. In fact, there is no general agreement on where to draw the line, but that line is certainly pink.
Scientifically, rubies and sapphires are simply varieties of corundum, a crystalline form of aluminum oxide (Al2O3), with impurities or trace elements such as iron, titanium, or chromium. These impurities are what create the wide range of colors to be found in corundum crystals: grays, browns, yellows, greens, blues, purples, reds, … and pinks.
Ruby is defined as “red corundum.” Chromium is what makes a corundum gemstone red. “All other varieties of corundum,” anything not red, are classified as sapphires. (Sapphires may contain a mix of chromium, titanium, and iron traces). Although sapphires are popularly associated with the color blue, they include all non-red colored corundum gems. It’s not surprising, then, that pink is the boundary line between rubies and sapphires since pink is commonly considered a tint of red (or red plus white).
Color can be quantified scientifically in terms of hue, lightness, and saturation. Recently, advances in colorimeters have also addressed the unique problems gemstone optical qualities like pleochroism and transparency have posed to standard color-order systems.
So why is there no general agreement on the difference between rubies and pink sapphires?
Although you can measure color, what one chooses to call a particular set of color measurements is still largely subjective. It comes down to the cultural and historical question: what is pink? In a fascinating article, Richard Hughes observes that before the 20th century, pink was considered a “light red” and some rubies were indeed described as “pink rubies.” Over the course of the 20th century, “someone decided that pink was not red” and the first references to pink sapphires appeared.
Professional gemology associations differ over the distinction between rubies and pink sapphires. Although the Gemological Institute of America (GIA) acknowledges historical and cultural variations on the division (or lack thereof) between red and pink color, it classifies as rubies only those corundum gems with “dominant” red hue. All other corundum is sapphire. On the other hand, the International Colored Gemstone Association (ICA) considers any red corundum gemstone, regardless of depth or intensity, a ruby.
Although sapphires have been highly prized throughout history, the demand for “blood red” rubies has been and continues to be exceptional. Gem quality rubies are rare, much rarer than diamonds. The desire to find, cut, sell, and own rubies has created a demand for a restrictive definition, which may account in part for the rise of the “pink sapphire” category. To return to the opening joke, that may be why buyers want to buy sapphires and sellers want to sell rubies.
However, the demand for pink sapphires has recently increased, according to David Federmen (who also challenges the argument that pink and red were ever considered the same color). “Today, pink sapphire needs no favors or apologies,” and “hot pink” sapphires are in demand, he writes.
Could an increasing demand for pink gems change the boundary between rubies and pink sapphires? Only time and the market can tell.