Five methods for enhancing the quality of rubies and sapphires through corundum treatments
In the captivating world of gemstones, few are as cherished as rubies and sapphires. These gemstones, known as corundum, have undergone various treatments to enhance their beauty and appeal. From heat treatment and dyeing to high-pressure high-temperature (HPHT) processes, these methods have played a significant role in the gemstone industry.
One of the most common treatments for rubies and sapphires is heat treatment. This process involves low to moderate temperatures and is used to improve the appearance of these gemstones by dissolving rutile inclusions or enhancing color. Infrared spectroscopy is a tool used by gemmologists to detect specific peaks, such as the 3309 cm–1 series, which can indicate post-growth artificial heat treatment.
Glass filling is another method used to improve the appearance of corundum. This involves filling fractures in the gemstone with a glass-like material. Glass-filled rubies are a notable example, where lead glass is used.
Though less common for corundum, HPHT can be used to alter the physical properties of some gemstones. However, it is not typically used for rubies and sapphires. Fracture filling, similar to glass filling, can involve different materials and is used to enhance the clarity by filling fractures.
Radiation treatment, while more common in other gemstones like diamonds and topaz, can be used to create new colors in gemstones. However, it is not typically used for corundum due to the potential for residual radioactivity.
Beryllium can be diffused into corundum, affecting the whole stone and producing a wide range of colors. Diffusion coloring elements can be diffused into white sapphires, resulting in coloration that requires repolishing. Fractures in poor-quality corundum may be hidden by filling them with oil or glass.
Titanium can be diffused into cabochon cut sapphires to improve a star. Irradiation may produce an unstable yellow color in sapphires, which can be detected only by a fade test. Evidence of the former presence of needle-like inclusions may remain as lines of tiny dot-like inclusions or small color patches along the directions of the inclusions.
The improvement of gemstones by treatments is acceptable, but buyers should be informed about any issues connected with durability. Heat-treating stones in contact with a flux, such as borax, may partly repair fractures. Since the 1970s, more sophisticated treatment methods have been used for corundum, improving the supply of gem-quality stones.
Work is carried out on nearly all gemstones before they are offered to consumers, including trimming, shaping, cutting, and polishing. Stones are often modified in other ways to make them more attractive, with a high proportion of corundum (rubies and sapphires) being treated.
Glass-filled rubies from Mozambique are recognized by iridescence, seen in fractures at low viewing angles, and a change of lustre when wider cracks intersect the surface. Heating can dissolve needle-like inclusions in corundum, improving the clarity of stones. Fractures often develop around other crystals, producing features called heat discs, and when temperatures reach about 1500°, previously transparent crystals may become whitish and translucent, taking on a "frosted" appearance.
A sapphire with abraded facet edges may indicate that it has been repolished after treatment. Those in the gem and jewellery trades should keep abreast of developments in this area of gemmology and consider sending doubtful stones to laboratories to confirm their status. Detailed microscopic examination, infrared spectroscopy, and specialized gemstone testing laboratories like GIA, IGI, and SSEF provide valuable insights into the treatments a gemstone has undergone.
In the end, while treatments enhance the beauty of rubies and sapphires, it is essential for buyers to be informed about these processes to make informed decisions.
- Gemmologists use infrared spectroscopy, a scientific tool, to detect specific peaks in the spectrum of heat-treated rubies and sapphires, which can help identify post-growth artificial treatments.
- Workshops focusing on various gemstone treatments, such as glass filling, diffusion coloring, and fracture filling, are crucial in the field of gemmology as they help enhance the appearance and durability of gemstones, including rubies and sapphires.
