**Preface to "Mineralogy and Geochemistry of Ruby"**

The studies in this volume specifically focus on ruby in its diverse connotations, which inevitably incorporate studies of allied minerals that accompany the presence of ruby in nature. This applies to the enclosing host rocks, mineral intergrowths with ruby, and the internal mineral inclusions within the ruby crystal structures. Some of these allied minerals provide radiometric elements for geochronological dating that helps constrain the ages of ruby growth, while others provide important clues to the geological and tectonic environments that formed ruby and affected its growth.

This Special Issue investigates one of the most sought after and valuable gem minerals within the annals of natural gemstone studies, namely ruby, the red-colored gem variety of corundum. The color results from a sufficiency of the trace element chromophore cation Cr3+, with modifications in color induced by other trace elements. Most of the contributions in this volume include invaluable data on the trace element variations that can be considered against color images of the studied rubies.

It also follows up on the publication of a preceding Minerals Special Issue on research advances that covered a wider spectrum of gemstones, "Mineralogy and Geochemistry of Gems" [1]. That volume included several studies on ruby. These are referenced here as useful supplementary studies for readers to consider [2–4], along with the studies presented in this Special Issue on ruby. During production of this Special Issue, other publications related to research topics on ruby continued to appear in the literature. One of special interest showed the extent that Cr can substitute into the ruby structure [5]. Ruby from secondary deposits from volcanic exposures at Mount Carmel, Israel, recorded contents up 32 wt % Cr2O3. These high levels were attributed to exceptional reducing conditions of formation, although the precise source of the ruby remains for further study.


### **Frederick Lin Sutherland, Khin Zaw**

*Editors*
