Gemstone Adventures: Mine for Moonstone and Learn to Set it in Jewelry

The recent full moon was so large and bright (earning itself "supermoon" status), so pretty in the spring sky, I found myself gazing at it as I traveled around Louisiana over the past weekend. Perhaps it's the gemstone training (or gem geekiness) in me, but every time I see a big full moon and that pretty glow it casts in the bright blue sky just before dark, I think of moonstones and the floating blue mist-like sheen that slips across their surface when they're turned in the light. That blue sheen is called adularescence, and only moonstones have it. Learn more about moonstones below and join rockhound Jim Landon on a moonstone-collecting adventure in Montana. Then learn how to cut your own moonstone cabochons and set them in stunning gemstone jewelry.

  MOONSTONE, 8 x 10 mm.
Peter Secrest Collection.
Photo by Jeff Scovil.

What is Moonstone?
The term moonstone describes a group of stones that display a lovely billowing effect, often best seen when cut en cabochon. They are all feldspars . . . a large and complex group, and we turn to Joel Arem's authoritative Color Encyclopedia of Gemstones for a more precise explanation of this gem.

"Moonstone refers to feldspar of widely varying composition and from a wide variety of localities [including India and Sri Lanka]. . . . Orthoclase moonstone consists of albite within an orthoclase matrix. A blue color is produced if the albite crystals are very fine; the sheen is white if the albite plates are thick. The color of the orthoclase may be white, beige, brown, red-brown, greenish, or yellowish. . . . Some of this material cuts fine catseyes, where the sheen is concentrated into a narrow band. The sheen in moonstone is referred to as adularescence."

Montana Moonstone: Visit a Little-Known Source of an Unusual Gem
By Jim Landon

When we arrived in southwest Montana at the beginning of last summer, my wife, Kerry, and I were greeted by hills ablaze with vivid green growth streaked with the purple of lupines. We had come to work on my seemingly perpetual cabin building project, as usual, but this year we were also going to hunt for moonstone in the rough terrain outside Bozeman where an acquaintance had a family claim. 

 
Moonstone cabochon. Photo by Jim Lawson.

After spending a wonderful Saturday night with Buzz and Patti Jones, jamming to good zydeco music and eating more food than we should have, we settled down with thoughts of our upcoming gem hunt. Buzz and I left early the next day, leaving our three dogs behind because of concern about rattlesnakes. The business of Bozeman quickly gave way to rolling fields of ripening wheat as we headed west toward the Madison River. We passed several road cuts with sediments resembling those of Oligocene and Miocene age I'd seen in western Nebraska. Sure enough, I later confirmed in Roadside Geology of Montana that they were Miocene and learned that they contained the remains of fossil horses, rhinos, and camels. As we continued west and entered the river valley, the geology quickly changed to the unfamiliar. The valley walls displayed twisted, tortured rock formations, evidently from past extreme tectonic events.

 

The steep cliff walls of this exposure are made up of ancient metamorphic rocks consisting of dense mica and garnet schists with numerous veins of quartz and feldspar. Photo by Jim Landon.

Mining in His Blood
Buzz pointed out a collapsed portal and one small old shack, the remnants of an old gold mining prospect that his family used to speculate about: just where was the vein the miners had been exploiting, and how close was it to their own claims? Buzz had also worked in a gold mine an uncle had a claim on and had learned to use a single jack drill at the age of 12-very young for such dangerous and back-breaking labor. Like so many native Montanans I've met, mining had long been a part of the Jones family: his great grandfather had first settled in the area in the 1800s, having traveled west from Iowa by wagon train. He'd started out farming and later developed an interest in ranching and mining.

After crossing the river, filled with people floating in all kinds of devices, a popular summertime activity here, we headed back downstream on an unimproved dirt road that paralleled the Madison. Here, the close-up formations looked like they were metamorphic rock, with large wavy bands of schists with bold stringers of milky quartz and what looked like pegmatites of feldspar and mica. They resembled formations I had seen in the Black Hills of South Dakota. Later, I again consulted my Roadside Geology of Montana and found that this area was part of a very ancient basement rock formation that had been subjected to extensive metamorphism in the past.

Then we left even the dirt road and started to follow a faint trail that headed toward the canyon wall, proof that Buzz's insistence on taking his rig and not mine was a good idea. This trail was not for the faint of heart, and definitely not a road for my extended cab Dodge pickup with the turning radius of an aircraft carrier!

 
Moonstone rough. Photo by Jim Lawson.

Near the site where their mining cabin once had been, I could just make out the access track to their claim that his cousin had developed. Bouncing around boulders and skirting around brush, we slowly made our way up toward the rock exposure where they'd found moonstone before. As we went through one hair-raising switchback, I found myself gripping the door and planning my escape if the truck were to take a header down the side of the canyon, but Buzz was calm as could be. When we finally reached the point where we could no longer go around the boulders, we came to a stop, much to my relief.

Look for the Flash
Sparkling with mica, the ground on our way to the diggings was littered with mica and garnet schists along with chunks of feldspar with books of mica imbedded in them. At the end of the path we found ourselves in front of a sheer wall, with irregular, milky-looking blobs imbedded in mica schist. It turned out that the milky-looking blobs were what we had come for: orthoclase feldspar (moonstone). The key was to find pieces that would display a blue flash when turned in the sun; those would be the pieces that had potential as cut moonstones. I quickly proceeded to work my way up the canyon wall through the brush to see what I could find.

 

The irregular feldspar crystals were interspersed among veins of biotite mica schist. Photo by Jim Landon.

About halfway up, I came upon what I thought was my first piece of moonstone. It glinted a faint blue in the sun and reminded me a little of Oregon sunstone I'd seen, only not as clear. Then a shout came from Buzz, and I made my way back down the slope, trying not to dislodge loose rocks that could pick him off and transport him into the river. He had found his first moonstone still imbedded in the cliff face, and when I positioned myself just right, I could make out the faint, deep-blue flash. The whole cliff face was full of distorted feldspar crystals, their crystal shapes not even discernible, all surrounded with swirls of biotite mica schist like so many nuts in pralines-and-cream ice cream, but I was at a loss about how to dig in. . . .

Read the rest of Jim's moonstone-hunting adventure in our gemstones eBook, The Complete Lapidary Experience: Hunt, Cut and Set Gems, where you can also learn to cut a moonstone cabochon and then set it in a gorgeous pendant. 

 

Learn more about using Mother Nature's beautiful treasures in your jewelry in Gemstone Settings: The Jewelry Maker's Guide to Styles & Techniques by Anastasia Young.

In Gemstone Settings, you'll discover great tips and information for using gemstones in your own jewelry designs, including how to set them, various types of gemstone settings, and jewelry design that starts with the gemstone and works into a design–plus basic jewelry-making techniques such as soldering, casting, wire wrapping, and more. If you love using gemstones in your jewelry making like I do, it's a great new resource for your jewelry bench! 

JIM LANDON is a long-time high school science teacher, rockhound, and budding jewelry artist who lives in the foothills of the Cascade Mountains in Washington State. Watch for more from him in coming months on Jewelry Making Daily!

 

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