A ghostly sheen moves under the surface of this feldspar, like moonlight glowing in water.
Moonstone’s delicate beauty and its long-established heritage make it perhaps the most familiar gem-quality member of the feldspar group.
Feldspars are the most widespread minerals in the earth’s crust, as well as some of the most diverse. You can pick up a rock anywhere in the world, and you’ll probably find that it contains a mineral or two from the feldspar group.
Moonstone is a variety of the feldspar-group mineral orthoclase. It’s composed of two feldspar minerals, orthoclase and albite. At first, the two minerals are intermingled. Then, as the newly formed mineral cools, the intergrown orthoclase and albite separate into stacked, alternating layers.
When light falls between these thin, flat layers, it scatters in many directions, producing the phenomenon called adularescence. Adularescence is the light that appears to billow across a gemstone, giving its surface a glowing appearance.
Perhaps the most captivating aspect of adularescence is its appearance of motion. The misty light seems to roll across the gem’s surface as you change the viewing angle.
Other feldspar minerals can also show adularescence. One is a labradorite feldspar found mainly in Labrador, Canada. Another labradorite—found in Madagascar—has a multicolored adularescence over a light bodycolor. It’s known in the trade as rainbow moonstone, despite the fact that it’s actually a variety of labradorite rather than orthoclase.
Sanidine is another feldspar mineral that can include adularescent gems called moonstones. To be called moonstone, a mineral’s actual identity is not as important as the beauty of its adularescence.
BIRTHSTONES & ANNIVERSARIES
Moonstone is a birthstone for June, along with pearl and alexandrite.
WHY WE LOVE THIS GEMSTONE
RARITY: Rare in top qualities, this ethereal gem derives from one of the earth's most common minerals.
ADULARESCENCE: Stacked layers of orthoclase and albite diffract light, creating moonstone’s adularescence.
ANOTHER PHENOMENA: Besides adularescence, moonstone sometimes also shows a cat’s-eye effect.
COLOR: Colorless to White, Gray, Green, Peach, Brown
REFRACTIVE INDEX: 1.518 to 1.526
BIREFRINGENCE: 0.05 to 0.008
SPECIFIC GRAVITY: 2.58
MOHS HARDNESS: 6.0 to 6.5