What is synthetic ruby?

Ruby's chemical make up is AL2O3:CR, that means that every molecule in it has 2 atoms of aluminum, 3 atoms of oxygen connected to an atom of chromium. In the 1800's chemists figured out that by combining aluminum oxide with chromium at high heat they would get synthetic ruby.
The process has been modified through the years but, chemically speaking, synthetic ruby is the same as natural ruby. From a retail point of view they are worlds apart and man-made stones must be marked as such and cannot be sold as ruby.