Ruby: Heat-Treating, Fracture-Filling, & Flux-Healing
Ruby (Corundum) Enhancements & Treatments
For over two thousand years rubies have been heated to enhance their reddish-pink color, and remove bluish hues. Heat treatment was accomplished using simple tools, such as a blow-pipe and charcoal burner.
Today, heat treatment of ruby is done using a combination of chemicals such as beryllium, borax, lead, and tantalum. One telltale sign of heat-treatment is the presence of small discoid fractures that appear around natural mineral inclusions. Heat-treatment temperatures range from 1500¼ to 1850¼ Celsius for a period of two to ten hours.
It has been reported by several reputable sources that virtually all of the newly-mined Mong Hsu rubies may have been heat-treated to remove the blue color cast [1, 2]. The Mong Hsu rubies tend to have a dark blue hexagonal zone running through the center of the stone. Burmese rubies may contain "fingerprints" and/or "feathers" after heat treatment.
Simulation of before-after heat-treatment
Occasionally, the stress of the heat treatment used to enhance color will create fractures known as "decrepitation feathers," which must then be repaired a process called "fracture filling." A high percentage of rubies on the market have been either heat treated, flux-healed or both. Small internal fractures that have breeched the surface of the cut stone are sometimes filled with a detectable process known as "glass-infilling."
According to the AGL (American Gemological Laboratories), up to 70 percent of Mong Hsu Burma rubies have been flux-healed or fracture-filled, with the work usually being conducted in Thailand. Thai mines in Chanthaburi and Kanchanaburi still produce small quantities of ruby, but most material coming from Thailand these days originated in Sri Lanka, Madagascar, or other ruby producing locations.
Lead Glass Treatments & Flux-Healing of Rubies
Surface cracks, cavities, or inclusions in ruby can be repaired using the "flux healing" (FH) method, also known as "glass in-filling." The flux-healing process involves exposing the stone to a combination of heat and solvents (borax and/or other fluxes) to fill any voids with molten low-viscosity flux "glass." As the flux mixture fills a fracture, it dissolves the walls of the fracture until the liquid in the crack becomes saturated with molten corundum/ruby solution (below, center).
When the molten flux mixture cools, the synthetic corundum will permanently fuse the crack together, but the process will leave behind small air pockets surrounded by solidified glass (above, right). These telltale signs are the characteristic signature of the healing process. Flux-Healing of fractures will reduce internal reflections making the ruby appear more transparent while permanently fusing the fracture together, making the ruby more durable. This bonding action differentiates flux-healing (FH) from fracture-filling (F).
Corundum Diffusion of Rubies
The process of corundum diffusion (aka lattice diffusion, deep diffusion) uses diffusion to transport coloring elements such as beryllium (Be) through the corundum to fill vacancies or voids in the crystal lattice structure. The lattice diffusion process is difficult to detect, and as such, is considered to be an unethical practice, and the diffusion treatment of ruby and sapphire continues to be an ongoing concern in the gem trade.
Synthetic corundum (ruby) was the first gemstone to be reproduced by artificial means using the "flame-fusion" method (aka Verneuil process), invented by a French chemist named Auguste Victor Louis Verneuil in 1902. Colorless synthetic sapphire called "diamondite," was also produced using the Verneuil process.
Synthetic ruby (corundum) created by flame-fusion (photo: Aram Dulyan)
The Verneuil process has since been replaced by the so-called "flux-grown" method which produces gem-grade ruby. Synthetic ruby can be identified with a microscope by observing its characteristic inclusions which can take the form of feathers, curved color-zoning, or bubbles which are all telltale fingerprints of its man-made origin. Inclusions of tiny hexagonal platelets originating from the walls of the platinum crucible are also a telltale sign of synthetic corundum.
Additionally, a loupe can detect the difference between the curved growth lines found in synthetic ruby corundum and the straight lines found in natural ruby corundum.
Sapphire Enhancements & Synthetic Sapphire
Burmese Mong Hsu & Mogok Ruby
Bibliography & Reference on Heating and/or Fracture Healing of Ruby
1. Richard W. Hughes, The Fracture Healing of Ruby www.ruby-sapphire.com
2. The Gem Forcaster, The Burmese Ruby and Sapphire Situation www.preciousgemstones.com
3. GIT, Clarity Enhancment of Ruby - 'New Treated Ruby Without Lead' www.git.or.th
4. G Du Toit, R Hughes, J Koivula. Beryllium-Treated Blue Sapphires . www.agta-gtc.org
5. Richard W. Hughes, John I. Koivula, The Synthetic Healing of Ruby . www.agta.org
Reference Credits & Further Study on Ruby Inclusions
6. Peter G. Read, The Gemstone Inclusion Library - Ruby . Canadian Institute of Gemmology
7. GRS, Ruby Inclusions Album . GemResearch Swisslab
Reference & Further Study on Rubies
8. Ted Themelis, Mogok: Valley of Rubies & Sapphires A & T Publishing, Los Angeles
9. Emporia State, Ruby and Sapphire - Varieties of Corundum . Emporia State University
10. Judith Osmer, Ruby and Sapphire . RWH Publishing
11. Judith Crowe, The Jeweler's Directory of Gemstones . DK Publishing.
12. Cally Hall, Gemstones . Simon & Schuster.
13. Renee Newman, Gemstone Buying Guide . International Jewelry Publications; 2nd edition
14. James E. Tennent, Ceylon Ruby, Sapphire & Gems . www.gutenberg.org
15. United Nations, Atlas of Mineral Resources of the ESCAP Region - Thailand . books.google.com