Ask A Gemologist

Over the course of a career teaching gemology, students have asked me many questions that gemologists tend to take for granted. This hit home recently at a writers group meeting when I casually used the term refractive index in one of my fictional works and the question immediately came up, “What is refractive index?”

This brief column is devoted to you, the curious student. It is an opportunity to ask anything you ever wanted to know about gemology (within reason) but were afraid to ask. So let’s begin.

What does refractive index (R.I.) mean?

Gem-A Refractometer

The amount to which a gem slows light is called its refractive index (R.I.). R.I. is calculated by the ratio of the speed of light in air to its speed in a gem. It is one of the measurable properties that gemologists use to assist with a gemstone’s identification.

For example, using your smartphone or computer, go to the computational database WolframAlpha. Type in “What is the speed of light in air?” The result is approximately 186,231 mi/s (miles per second). As light enters a transparent material it slows down based on how optically dense that material is. The speed of light in diamond is measured at 77,050 mi/s. When you divide the two you get the R.I. for diamond (2.417). This tells me that light travels 2.417 times faster in air than in diamond. Or light slows down 2.417 times because of diamonds optical density. Gemologists use a refractometer to measure this difference. However, instruments do have limitations.

What is Transvaal jade?

Transvaal jade, known in gemology as Hydrogrossular Garnet, may appear colorless, white, yellow, brown, red, or green. Greenish hydrogrossular garnet, though only outwardly resembling jade, is sometimes marketed under the name South African, or Transvaal, jade in an attempt to increase its selling price.

Typically green to bluish green. This translucent material may exhibit black, pepper-like inclusions. A skilled observer will look for a vitreous (glass-like) polish luster, uneven, granular, or splintery fracture with greasy to vitreous fracture luster. If you’re able to measure it’s refractive index (R.I.) typically you would got a 1.72. Nephrite or jadeite gives a solid 1.62 / 1.66 R.I. respectively. Practiced Jade experts identify quality jadeite simply by appearance and lustrous feel. See “A Jades Story”, Winter 2013 edition, WJA News San Diego.

If you’re a curious student and would like to ask questions please e-mail pball@sbcglobal.net. Or you can always find me on Twitter at pball001, on Facebook, or my blog, http://pball001.blogspot.com.

“Satisfaction of one's curiosity is one of the greatest sources of happiness in life.” ~Dr. Linus Pauling

(This post appeared in Summer 2014 edition, WJA News San Diego)

Comments

On Becoming Real

Some things get better with wear: old shoes, old friendships, and old books. Especially old books. A new volume arrives with crisp corners and a spine that hasn’t yet learned to bend. An old one has been somewhere. It has traveled through time in the hands of people who may no longer be around to tell us what they found inside. The copy of The Velveteen Rabbit (first published in 1922) on my desk is one of those books. My friend Don Hanley gave it to me. This late-1950s Doubleday edition features William Nicholson’s original illustrations and cream paper boards softened by age. Worn corners, a split spine, and colored tabs mark passages worth revisiting. Don didn’t hand me a keepsake. He gave me homework. When we talked about the book recently, he pointed me straight to the nursery conversation between a young rabbit and an old Skin Horse. The rabbit had noticed the mechanical toys around him—the ones with internal gears, bright paint, and stick-out handles that seem terribly impressiv...

Engineering Beef Stew

On July 2nd—Day Two of what I call The Great Un-Working —I sat down with my morning notebook to map out this new chapter of my life. One goal made the list in bold ink: Learn to Cook. At seventy, I was adding a skill most people master before they’re old enough to drive. Before you judge me, understand our household ecosystem. Lori is a phenomenal cook. For decades, she’s made meals so effortless and delicious that restaurants seem unsatisfying . My contribution to the culinary arts during my bachelor years–tuna mixed into Kraft Mac & Cheese. Let’s just say I wasn’t threatening her spot on the Food Network. But retirement changed the equation. I’m officially off the clock while Lori is still working. And I had a lovely thought: Wouldn’t it be wonderful if she came home after a long day to find dinner waiting? She loved the idea. Then came the question that haunted me for weeks: “What do you want to make?” I had no idea. Apparently, the first step in learning to cook is deciding wh...

When the Universe Calls in Sick

Take Thoreau’s Advice . . . Four weeks ago, we stood on the shore of Walden Pond in Concord, Massachusetts, trying to channel Henry David Thoreau. Before leaving, I bought a pocket-sized copy of his essay, Walking . It’s slim because nineteenth-century transcendentalists didn’t have notifications interrupting their urge to wander. Thoreau’s counsel is disarmingly simple: step away from modern life's machinery and let the landscape recalibrate your perspective. Back home in Vista, I took that philosophy straight into the backyard. Thursday morning, ink-black, pristine skies arrived. I aimed my refractor at Jupiter, whose light has bathed Earth since mastodons roamed the plains. It felt cosmic. Profound . I left the telescope mounted overnight, convinced the heavens were on retainer for an encore at daybreak. Naturally, the universe called in sick. By sunrise, a wall of coastal fog had rolled in off the Pacific, sealing the valleys beneath a damp, gray Tupperware lid. After about th...

The Gopher Problem and the High Powered Solution

Vacationing in Maine this week, just a short drive down the coast from Freeport, it is hard not to think about the outsized impact one frustrated outdoorsman had on the gear we all use today. Bad equipment can ruin a perfectly good day on the water or the trail, but it takes a special kind of vision to turn cold feet into a century-old empire. This is the story of the company we now know as L.L.Bean, but more importantly, it is a masterclass in integrity, learning from failure, and standing behind your work. In 1912, Leon Leonwood Bean faced a persistent problem: returning from expeditions in the Maine woods with cold, wet feet. Instead of complaining, he got creative. He had a local shoemaker stitch waterproof rubber soles onto the flexible leather uppers of a hunting boot. He called it the Maine Hunting Shoe and mailed a flyer promising perfect satisfaction. Then reality arrived. Ninety of his first 100 pairs failed when the rubber separated from the leather. Not an effective product...

On The Fly

Search This Blog