gems, inclusions, feldspar, orthoclase, moonstone, sunstone

Gübelin and Koivula examine feldspars both as gemstones containing inclusions and as mineral inclusions occurring within other gemstones. Using photomicrographs, they illustrate characteristic internal features of gem feldspars—including members of the orthoclase and plagioclase groups—and examples in which feldspar itself occurs as an included mineral. The article emphasizes the value of identifying such inclusions for understanding the geological environment and formation history of their host gemstones, while demonstrating the considerable variety of microscopic features associated with the feldspar group.