corundum, inclusions.5, Montana.12, USA.12, gems

This article offers a brief description of the history and geology of the Yogo sapphire deposit the largest producer of gem sapphires in the Western Hemisphere. Until recently, the mine was largely inactive, and the factors leading to this period of low production and the associated economics of the deposit are discussed. Recent discoveries at the deposit and the development of a new mine in a sapphire rich zone mean that Yogo sapphires may become more well known than in the past. The gemology of this relatively inclusion-free and typically untreated gemstone is also discussed. Photographs of some of the inclusions that do exist are presented.