Today I’m going to discuss the similarities and differences between labradorite, moonstone, and rainbow moonstone. I’ve noticed that most customers who ask for moonstone jewelry really mean rainbow moonstone, they just don’t know it!

All three of these gems are part of the feldspar group of silicate minerals that have a hardness of 6 – 6 1/2 on the Moh’s scale. Feldspar is a very common rock forming mineral, but it has a range of chemical composition leading to many different varieties of minerals in this group.  Within the chemical formula of feldspar, several different elements can be substituted in part or completely by another element. Only a few of the many different feldspar minerals are rare or beautiful enough to be considered gems.

All three gems show “phenomena”; special optical effects due to how they transmit and reflect light.  Read more about the different phenomena mentioned below in these blog posts: Part 1, Part 2.

Labradorite:

Labradorite pendant and beads showing labradorescence

Labradorite is classified as a plagioclase feldspar with the chemical formula (Ca,Na)[Al(Al,Si)Si₂O₈]. This formula shows that there can be some variation in the relative proportions of calcium and sodium, and aluminum and silica, and still be classified as labradorite.

Labradorite was named for where it was discovered, Labrador, Canada. What makes this gem special is the phenomenon of “labradorescence” (great originality in naming). Without direct light, the gem generally appears gray. In the light, vibrant blues and greens flash across the surface. Sometimes gold, pink, or other colors are also seen; the name “spectrolite” is sometimes used in this case.

Moonstone:
Moonstone is the gem form of the mineral orthoclase (potassium feldspar, sometimes

Some of the colors of moonstone.

called K-spar) with the chemical formula ‎KAlSi₃O_8.  

Common colors include white, gray, and orange. Less commonly seen are yellow or brown. Moonstone’s phenomenon is called “adularescence” which gives the gems a whitish sheen or cloud that appears to float within the stone. This sheen, which appears to some to look like the moon shimmering in the sky, is the source of the name.

Rainbow Moonstone:

Rainbow moonstone (labradorite).

Here’s the chemical formula for rainbow moonstone: (Ca,Na)[Al(Al,Si)Si₂O₈]. It’s exactly the same as labradorite because … rainbow moonstone is actually a variety of labradorite!

Rainbow moonstone can be transparent to translucent white. Rainbow moonstone also shows adularescence, but the sheen is blue, sometimes with other colors, and is caused by the same mineral structure that causes labradorescence.

All three gems are feldspars and all are beautiful – their differences are just a matter of chemistry. My favorite of the three is labradorite. What’s yours?

References
GIA Gem Identification Lab Manual, May 2012.
Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York.
https://www.mindat.org
https://www.gemdat.org
https://www.gemselect.com

All photos © J. Fenton, Mochi’s Gifts, 2018