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 1500º to 1850º Celsius for a period of two to ten hours.
Virtually all of the newly mined Mong Hsu rubies are heat-treated to remove the blue color cast (before and after, below). 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.
Occasionally, the stress of the heat treatment used to enhance color will create fractures ("decrepitation feathers") which must then be repaired. A high percentage of rubies on the market have been either heat treated, flux-healed or both. Small surface fractures are sometimes filled with a detectable process known as "glass-infilling." According to the AGL (American Gemological Laboratories), 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 elswere.
Lead Glass Treatments & Flux-Healing of Rubies
Surface cracks, cavities, or inclusions in ruby can be repaired using the "flux healing" (FH) method. 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 Ruby
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.
The Verneuil Process has been replaced by the Flux-Grown method which produces high-quality gem-grade ruby. Synthetic ruby can be identified with a microscope by observing its characteristic inclusions or feathers that are fingerprints to its origin. Inclusions of tiny hexagonal platelets originating from the walls of the platinum crucible are 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.
Reference Credits & Further Study on Heating or Fracture Healing of Ruby
Richard W. Hughes, The Fracture Healing of Ruby 
www.ruby-sapphire.com
The Gem Forcaster, The Burmese Ruby and Sapphire Situation www.preciousgemstones.com
GIT, Clarity Enhancment of Ruby - 'New Treated Ruby Without Lead' www.git.or.th
G Du Toit, R Hughes, J Koivula. Beryllium-Treated Blue Sapphires . AGTA Gemological Testing Center
Richard W. Hughes & John I. Koivula, The Synthetic Healing of Ruby . American Gem Trade Association
Reference Credits & Further Study on Ruby Inclusions
Peter G. Read, The Gemstone Inclusion Library - Ruby . Canadian Institute of Gemmology
GRS, Ruby Inclusions Album . GemResearch Swisslab
Reference Credits & Further Study on Rubies
Ted Themelis, Mogok: Valley of Rubies & Sapphires A & T Publishing, Los Angeles
Judith Crowe, The Jeweler's Directory of Gemstones . DK Publishing.
Walter Schumann, Gemstones of the World . NAG Press; 2Rev Ed edition
Renee Newman, Gemstone Buying Guide . International Jewelry Publications; 2nd edition
Antoinette L . Matlins, Antonio C. Bonanno, Gem Identification Made Easy . Gemstone Press; 3rd edition
Paul R. Shaffer, Herbert S. Zim, Raymond Perlman, Rocks, Gems and Minerals . Martin's Press
Vincent Pardieu, Jean Baptiste Senoble, Ruby Mining in South East Asia . www.fieldgemology.com
Richard W. Hughes, Ruby and Sapphire . RWH Publishing
Emporia State, Ruby and Sapphire - Varieties of Corundum . Emporia State University
Judith Osmer, Ruby and Sapphire . RWH Publishing
