Although sunstone and moonstone are both members of the feldspar group, the resemblance stops there. While cool moonstone’s phenomenal varieties offer a soft and gentle adularescent glow, warm sunstone’s phenomenal varieties show a distinct and lively glitter called aventurescence.
Aventurescence is a sparkly, metallic-looking luster caused by flat, reflective inclusions. Although most sunstones have yellow, orange, or brown body-color, not all sunstone is aventurescent. The appearance of the phenomenon depends upon the size of the inclusions. Small inclusions create a reddish or golden sheen on top of any bodycolor. Larger inclusions create attractive, glittery reflections. Sunstone inclusions can be hematite, copper, or some other mineral.
Not all feldspars that bear the name “sunstone” are from the same side of the feldspar family—both the orthoclase and the plagioclase species boast a sunstone variety of feldspar.
Orthoclase sunstone belongs to the monoclinic crystal system. While Oligoclase sunstone is plagioclase feldspar that belongs to the triclinic crystal system.
The name sunstone refers to the gem’s appearance rather than to its chemical makeup. There are many sunstone varieties. If aventurescence is present, gemologists call it aventurine feldspar. The aventurine feldspar from India has a red-brown body-color and sunny glitter. It’s perhaps the best-known sunstone variety.