Hunting for peridot EVERYWHERE, the birthstone for August

Sunset Crater was too hot! There were no trees (or bushes, or grass), too much dust and I had a rock in my shoe. I was walking around the base of a giant pile of bits of red cinder of the sort I had previously encountered in xeriscape gardens looking for olivine. Even the breeze was hot. And worst of all, I didn’t even know what I was looking for. Olivine? We had been informed the bright orange and red cinder cone had bright green gemstones and then our class had been turned loose to hunt for olivine at the youngest of the San Francisco Peaks in Arizona. I sighed and gave up. There was NOTHING green out there and my time would be better spent taking pretty pictures. I didn’t realize at that moment that I would spend a lot of my adult life looking for and at olivine, otherwise known as the gemstone Peridot: the birthstone for August.

Around base of Sunset Crater, from the Arizona Geologic Survery

I kind of stumbled into geology. It was the end of my first year at junior college and I had concluded that I did not want to be an engineer after all. And in doing research for what I DID want to be I had also rejected journalism, history, art, physics, anthropology, English, nursing, physical therapy and kindergarten teacher… and been rejected by chemistry, which I had thoroughly enjoyed. My chemistry teacher said my only future as a chemistry major was in the bomb squad! I was beginning to run out of majors to investigate when my mother suggested I go on a two-week geology field trip. Sunset Crater was towards the beginning of the trip; by the end I was a geology major.

Despite being a geology major, my next attempt at hunting for olivine a year later in old lava flows on Hawaii, didn’t go any better. I stared at the giant black cliffs of basalt lava and tried to identify minerals. Plagioclase = white blocks and needles. Black holes = vesicles. Red things that keep moving = rock crabs. Olivine SHOULD be little roundish bright green gemstones. Nope, no olivine. I looked the whole trip, heard people wax poetic about green beaches of olivine that I couldn’t find and I never saw any olivine….

Despite handily identifying it in lab classes, I had to start my senior thesis before I saw olivine someplace other than jewelry, a classroom or photograph. By the time I was doing my Master’s thesis, I had become accustomed to finding olivine in the basalts I studied. And I had learned why I had never seen the olivine. Olivine is a shiny, roundish gemstone that is a bright, clear, yellowish green. It is found mostly in basalts, black lava in places like Hawaii, the Snake River Plain in Idaho and cinder cones and lava flows in Arizona. It is REALLY common! But, it has two problems if you are an inexperienced rock-identifier. The first, is that olivine tends to form relatively small crystals (1/8 inch or smaller!), so it is easy to overlook. The second is that olivine weathers or rusts to a red color and then to clay and dust relatively quickly, as gemstones go. This doesn’t mean you should expect your jewelry to disintegrate, but does mean the thousands of years-old lava flows in Hawaii have holes and red smudges where the olivine should be.

It was in the middle of my Master’s thesis, on a different trip to Hawaii with my family, that we suddenly tripped over a beach composed of boulders of picrite. Picrites are basalts that are composed of LOTS of olivine (and a few other things). After all that searching I had found big, pretty crystals of olivine completely by accident! I was overjoyed and immediately started making plans to take samples home and hunting for the perfect rocks...

A whole beach full of olivine!

Giant rocks on beach

One or so for me. One for my thesis advisor, because I needed to bring him omiyage (a travel gift) and a rock would be perfect. One for a friend who would think it was pretty…. The rocks needed to have LOTS of olivine (because that was the point) and be the right shape and size. The boulders were all lovely and rounded in the surf, but the average size of the rocks on the beach was HUGE. As in, I would need a forklift to carry a rock away huge. So I scrambled here and there (while my family watched in amusement) comparing and lifting rocks until I had narrowed it down to about 50 pounds of possible rocks. I did narrow it down from there, but no one was surprised when my suitcase was overweight at baggage check-in. Agricultural check didn’t even blink at the contents of my bag (they did have a moment of confusion with a wet t-shirt my sister had rolled up to take home; apparently it looked like a mango).

Home again, a baggage cart handler couldn’t understand the chorus of laughter when he asked, “Wow! What do you have in here? Rocks?”

The beach stones I kept for myself.

I sell olivine in my shop! Come see!

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