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

The Language of Home: Building a Sanctuary

This episode is for anyone trying to find their footing in a new place—whether it’s a new city, a new job, or a new country. The light in Florence, Italy, has a way of making everything feel like a Renaissance painting—the golden hue on the stone, the steady rhythm of the Arno River, and the feeling that you are walking through a history much larger than yourself. I was there to give a presentation to a class of Gemology students. I was prepared to discuss color grading and refractive indices, but not to be outed as a language tutor . Feeling very much like a guest in a storied land, a hand shot up enthusiastically. "You’re the guy on the podcasts," the young woman said, her eyes bright with recognition. "You’re the one teaching us English." I laughed nervously. If you know my flat Midwestern accent, you know the irony here. I am hardly an Oxford professor. But later, as I wandered the cobblestone streets beneath the shadow of the Duomo, the humor faded into a powe...

Practiced Hands: The 50-Year Warranty

What Doc Burch Taught Me About Staying Active. We talk a lot about "life hacks" these days, but most of them don’t have a very long shelf life. Usually, they’re forgotten by the next app update. But back in 1972, I received a piece of advice that came with a 50-year warranty. It’s the reason I’m still on my bike today, still chasing a golf ball around Carlsbad, and still—mostly—in one piece. The Kick That Changed Everything It started with a literal kick in the pants. A kid at school in Cuba, Illinois, was joking around and caught me just right. By the next morning, my lower back was screaming. My mom didn’t reach for the Tylenol; she reached for her car keys. "Let’s go see Doc Burch," she said. "He’ll fix you right up." Harry E. Burch, D.C., was a fixture in Lewistown. He’d graduated from Palmer College in ’59 and had been our family’s go-to for years. He was a man of practiced hands and steady eyes. After a quick exam and an X-ray, the mood in the room s...

Chasing 70

In this episode, Chasing 70: A Respectful Negotiation with Gravity They say golf is a game of misses. If that’s true, my first round of the year at Rancho Carlsbad was a masterclass in missing efficiently . After a four-month hiatus—during which my golf clubs quietly evolved into a self-sustaining garage ecosystem—Lori and I returned to our local par-three proving ground. Rancho Carlsbad is a par-54, just 1,983 yards long. That sounds forgiving until it exposes every weakness you’ve been politely ignoring during the off-season. I finished with a 78. In most contexts, 78 is respectable. On a par-54, it means I spent a fair amount of time “getting my steps in.” But here’s the real motivation: I turn 70 this August. As a core principle of my Great Un-Working Lifestyle, I’m putting it in writing: I want to shoot my age by my birthday. The Bald-Headed Man Course Around here, we have a nickname for Rancho Carlsbad. We call it the Bald-Headed Man Course. First, because there are no woods...

The Miller Effect

In this episode - The Miller Effect . . . The sun hung high in the sky, casting shadows across the desolate landscape of Huron, California. Dr. Vo, a brilliant yet witty electrical engineer, stood before the main breaker box of a massive 1.4 MW-DC solar array that had confounded everyone who had dared to diagnose its persistent issue. It had been six long months of head-scratching and ten failed attempts by others before the desperate call came into Dr. Vo's office. As the sun's rays bathed the vast array in an orange glow, Dr. Vo stepped up to the Main breaker box, his sharp eyes shaded by his green Cenergy cap. He wore his North Face jacket that billowed in the light breeze, and his presence exuded an air of mystery and intrigue that was as pervasive as the problem at hand. The solar array was a colossal assemblage of panels, wires, and inverters, but the main breaker kept tripping, sending the entire operation into chaos. The workers at the site were on edge, muttering, “We’...

On The Fly

Search This Blog