is the goup of rubys and rubys and sapphires.
Just aluminium oxide with the chemical structure Al2O3.
Colorgiving substances are titanium, iron, vanadium and chrome.
There are several optical effects: colorchange, catīs eye, asterism in 6 or 12 ray stars, or a diffuse misty shine above the surface which has a hardness of 9.
Every ruby is corundum, but not every red corundum is ruby, it has to be colored by chrome.
Sapphire can have any color and without optical instruments itīs not possible to divide red sapphires from rubys.
Since more than 100 years there are synthetic corundums in gemstone quality on the market.
Because of their high price natural corundum have been treated and tricked wherever possible, the latest idea was to glasfill worse synthetic rubys to declare and sell them an clarity improved natural stone.
To detect the filling you need at least a good magnifier lens to see air bubbles and "blue flashs" inside along the cracks.
The best instrument for detection of most treatments and synthetics is still a gemmological microscope.
Glasfilling itsself is no bad idea, emeralds are oiled for hundreds of years to improve clarity, peole use anti aging cream to improve their surface, so why not fill small cracks in corundums with glass, it tunes clarity and surface and lasts much longer than oil or cream.
The Stone just has to be declared as trated that way.
That way you can get a nice big ruby for a fraction of the price a similar stone without make up would cost.