What is milky, grayish, or brownish corundum that can be treated to a fine blue color known as?

The term "Geuda" refers specifically to milky, grayish, or brownish corundum that can undergo treatment to achieve a beautiful blue color. This type of corundum is often found in deposits that may not initially display the desired clarity or color, making it less valuable in its natural state. However, through a treatment process, especially heat treatment, Geuda can be transformed into sought-after gemstones like blue sapphires.

On the other hand, corundum is the mineral species that encompasses both sapphires and rubies but does not specify the particular qualities or treatment aspect associated with Geuda. Spinel is a different mineral entirely and isn’t related to corundum's potential for treatment. Ruby, while a form of corundum, specifically refers to the red variety and does not apply to the milky or grayish tones that Geuda describes. Hence, the appropriate name for the described material is "Geuda."