The Verneuil process is a flame fusion process for the manufacture of synthetic sapphires.
Very fine, pure aluminium oxide is melted with an oxyhydrogen torch (oxygen + hydrogen) at a temperature of more than 2000 degrees.
This material falls drop by drop onto a crystalline seed which grows continuously like a stalagmite and thus gives rise to a crystal.
The production process takes from a few hours to several days. In order for the crystal to grow through the superimposition of very thin layers of molten material, the melting process is carried out continuously in the same temperature zone by a downwardly directed flame.
The Verneuil process produces only small quantities of coloured rubies, sapphires and spinels. Most of it is grown as colourless crystal, but increasingly as raw material (crepites) for growing sapphire by means of other manufacturing processes.