You can see on the picture beside, a boule being started.
Verneuil was the first to produce corundum single crystal (sapphire) of useful size around 1900. The modern flame fusion process still bears his name. It has retained most of its original characteristics: an oxygen hydrogen diffusion flame provides the high temperature necessary for melting the top of the growing crystal, i.e. 2050°C; and alumina powder is fed slowly through the flame, melts in the flame , and deposits at the top of the crystal. The “boule” is then continuously lowered so that the solid-liquid interface remains at a constant height in the flame.
The pure alumina powder is used to produce colorless crystals. However, by adding minute amounts of selective metal oxides impurities to the feed powder, colored crystal are obtained: chromium oxide addition produces ruby, titanium and iron additions produce blue sapphire.