What are white sapphires?

To try and better explain what a white sapphire is, lets first explain a little bit about sapphire itself. 

Sapphire and Ruby are both Corundum (or its actual elemental composition is Al₂O₃), it has a hardness of 9 on the Moh's scale (for reference, diamond is a 10, emerald a 7.5-8, and Citrine a 7).

Most people normally think of sapphire as blue, it gets this blue color from varying amounts of iron and titanium impurities in the crystal, lower concentrations providing a paler stone while heavier concentrations providing a deeper richer color.

Ruby being the same material gets its color from trace impurities of Chromium and Iron. Starting to see the trend here?

As nature varies the trace impurities in the corundum, this also causes the many color variations that you find with a natural sapphire. Now what would happen if this trace impurities were minimal or not present in the corundum?

White  (or sometimes referred to as Colorless) Sapphire