Sunstone is a member of the feldspar group. Both the orthoclase and the plagioclase feldspar species boast a sunstone variety. Other feldspar group gems include moonstone, non-phenomenal orthoclase, phenomenal and non-phenomenal labradorite, and amazonite. Sunstone from Oregon is gaining attention as a natural and untreated product of the United States.
Gem connoisseurs and innovative jewelry designers often seek the subtle beauty of lesser-known—but still intriguing and unique—gemstones. Some of those are members of the feldspar group.
The feldspar group’s wide variety of unusual gems includes moonstone, non-phenomenal orthoclase, phenomenal and non-phenomenal labradorite, sunstone, and amazonite. Of these feldspars, moonstone is a constant favorite, while sunstone from Oregon is gaining attention as a natural and untreated product of the United States.
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, sometimes called “schiller” by sunstone fanciers.
The feldspar group has many members. Some are suitable for jewelry use, some aren’t. Members of the feldspar group vary slightly in chemical composition, and those variations produce a variety of gemstones that differ widely in appearance.
There are two main branches of the feldspar family tree—feldspars that contain potassium and feldspars that contain a mixture of calcium and sodium.
Two species of feldspars contain potassium. These are orthoclase, which includes the gem varieties moonstone and orthoclase sunstone; and microcline, which includes the gem variety amazonite. A non-phenomenal form of orthoclase feldspar appears in jewelry on rare occasions. Sometimes faceted, its hue is usually a transparent yellow.
Plagioclase feldspars contain a mixture of calcium and sodium, and they have slightly different compositions. The gem species in this feldspar family include labradorite and oligoclase.
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 feldspar variety. 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 bodycolor and sunny glitter. It’s perhaps the best-known sunstone variety, but that situation has been changing.