A symbol of love and passion, rubies are perhaps the most prized variety of the mineral corundum. Traces of chromium found in the ruby's crystal structure cause its red color, which can range in shades from orange-red to a purplish red.
In their rough form, rubies often have needle inclusions and color impurities. Therefore, nearly all stones are treated in some way to enhance their color and transparency. A few of the most common methods include heat treatment, fracture filling, lead-glass filling and lattice diffusion.
Traditional heat treatment is widely accepted within the gem trade. Submitting rubies to intense heat maximizes the purity and saturation of their red hue. Natural rubies often contain impurities such as purplish coloration which interfere the intensity of the red color. Heating has the effect of eliminating the purplish impurity, thereby accentuating the red hue. A secondary effect of heating is that minute needle-like inclusions are dissolved; this makes the stone to appear lighter in tone. Sometimes heating can cause recrystallization of silk inclusions, making them more prominent. This renders a sharper asterism, or reflecting star effect, within the treated ruby.
In some cases, a substance is used to fill small surface-reaching fissures in rubies. This method, known as fracture filling, has the effect of reducing the visibility of the cavities and improves the appearance of the gem. The filling materials include glasses, oils, resins, and waxes. Glasses are quite harder thus more durable compared to other materials. Although fracture filling increases the stone’s clarity and color, it doesn’t guarantee an exceptional durability. Exposure to harsh chemicals and heat can damage the fillers that are typically used. Therefore, special care is required when cleaning rubies.
Lead-glass filling is a recent innovation in concealing cracks in rubies. Low quality rubies with significant fissures in them are filled with a relatively large quantity of lead glass to fill voids or fractures. The high percentage of lead-glass accounts for much of the gem’s weight, thus rendering the gem at a lower value than natural or traditionally treated stones. Performed at a lower temperature, lead-glass filled rubies or composite rubies, are much more fragile than natural or traditionally treated stones. Rubies treated this way need special care and this should be disclosed by the seller. Consumers considering the purchase should not confuse lead-glass filled rubies with traditionally treated stones containing a small amount of glass.
Lattice diffusion is a recently invented method that requires an infusion of foreign elements into the atomic lattice of the stone. Rubies are typically exposed to a very high temperature in the presence of color-inducing component known as beryllium. This element has smaller atomic structure than titanium or chromium, so it is able to diffuse all the way through the rubies, effectively accentuating their color. The treatment is considered permanent.