SEEING RED: RARE RUBIES FROM BLUE SAPPHIRE PRODUCING DEPOSITS AND GEOCHEMICAL CONSTRAINTS ON CHROMIUM ENRICHMENT

Many gemstone deposits around the world can be neatly compartmentalized into simple categories. Colombia produces emeralds, sapphires come from Australia, Mozambique is known for its rubies. Such simplifications can allow for more facile understanding of the nature of gem deposits. However, sometimes nature doesn't play along with such schemes. The gem island of Sri Lanka is known for producing the legendary blue sapphires of antiquity. But many other gems are unearthed here, even including rare rubies. There is Mogok, famous its rubies, but also renowned for its fine blue sapphires. Fortunately, our failure to neatly compartmentalize nature can sometimes provide interesting insights. This contribution will focus on gem mines known for blue sapphires, but which produce rare rubies in an attempt to understand their geological conditions of formation.

This study includes blue sapphires and rubies from Rock Creek, Missouri River, and Yogo Gulch, Montana as well as from Sri Lanka. The basic answer as to why some deposits produce primarily blue sapphire and not ruby is of a geochemical nature. Rubies are red due to Cr, and so the geochemical environment that produced the blue sapphires must not have supplied enough of this chromophore to affect these stones' color. However, comparing the trace element profiles of rare red rubies and blue sapphires shows almost no correlation between Cr and any of the other trace elements. The geochemical conditions that controlled the Cr content of these stones must have operated independently of the other trace elements. However, the similarity in their trace element profiles (besides Cr) suggests an overall similarity in their formation conditions. This hypothesis is supported by observations of the inclusion scenes in the blue sapphires and associated rubies. The stones from a single deposit tend to have very similar inclusion scenes regardless of their Cr content. While much is still unknown, the general observations provided here may help to start unraveling the geological histories of these deposits.