According to the Encyclopaedia Britannica, rubies are formed when the mineral corundum is exposed to chromic oxide in metamorphic environments between 1148 to 1238 degrees Fahrenheit. There are also two prominent methods of making synthetic rubies in a laboratory. The flame-fusion method produces a gem with notable imperfections. The flux-growth method produces a synthetic ruby that is nearly indistinguishable from a real one.
Rubies are made from corundum or aluminum oxide. The corundum attains the distinctive ruby red color when chromium displaces some of the aluminum ions in the aluminum oxide; this process is called isomorphous replacement. Conditions of extreme pressure and heat in which hydrothermal fluids meet limestone are needed to create the metamorphic environment necessary to create rubies.
The flame-fusion method of creating rubies in a laboratory involves melting the essential elements and dripping the mixture onto a boule.The stone crystallizes in just a matter of hours. Because this technique leaves tiny gas bubbles and a glassy, unnatural appearance, the rubies it produces are inexpensive and mostly used as costume jewelry.
In the flux-growth method, jewelry makers melt the elements into a molten mix called a flux. The crystal takes six months to form under controlled-pressure conditions. Even their flux-rubies are not real, their authentic appearance gives them a high price -- up to $500 per carat.