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

Paris – the End of Silence

✈️ In this special episode: Paris – the End of Silence Sometimes, connection arrives in the most unexpected form—not through grand gestures, but through a quiet voice carried by technology. In a Paris apartment, I finally understood my family’s words . . . and felt my mother’s presence in every sentence. Since I was a little boy, France has been both a beautiful and frustrating paradox in my life. Every six to nine years, my mother, Mauricette, would take my brothers and me back to La Rochelle to visit our French family. The moment we arrived, the air would fill with a sound I loved but couldn’t share in—the rapid-fire, musical rhythm of French. My aunts, uncles, and cousins would warmly sweep me into hugs and kisses, their words flowing like a lovely melody I couldn’t quite catch. I’d smile brightly, trying to communicate with my eyes and hands. But as soon as we stepped off the plane, my mother and her sister-in-law, Joséan, started talking animatedly. They were gone, chatting h...

Noirmoutier: An Ocean Between Us, Gone in a Moment

In this episode, Noirmoutier: An Ocean Between Us, Gone in a Moment. Sometimes love waits half a century for its moment — and when it finally arrives, time doesn’t stand still; it disappears. The moment I stepped off the train in Nantes, it felt like time froze. There she was — my cousin Michèle — waiting on the platform, arms waving desperately. When we finally embraced, the fifty years that had passed between us disappeared in an instant. The melody in her voice was the same, but softer than I remembered. We both shed tears of joy that only come from love long overdue. “I’m so happy you are here,” she whispered, her voice trembling. Thank goodness for the translation app on my phone, because the conversation began immediately — fast, fluid, and unstoppable. The Frenzy of Catching Up As we drove for about an hour to the tiny town of L’Épine on the Island of Noirmoutier, the words kept tumbling out. Michèle and her husband, Alain, are the most gracious hosts — but my new challe...

Patience – the Only First-Class Ticket

In this episode, Why Patience is the Only First-Class Ticket They say travel broadens the mind. After eight days sailing the Rhône with 140 fellow luxury vacationers, I can confirm it also tests patience, calf strength, buffet strategy, and one's tolerance for people furious that France insists on being French. Don't get me wrong—I adored this trip. The river shimmered like liquid optimism. The villages looked hand-painted. The pastries could negotiate world peace. But somewhere between Ship Horn Hello and Bon Voyage, we'd inadvertently boarded a floating behavioral research study disguised as a holiday. Our ship was less a cruise and more a ferry for the Sailors of Status. ⌚ The Wristwatch Wars Some passengers approached relaxation like yogis. Others treated leisure like a final exam with extra credit. I came to believe certain luxury watches emit ultrasonic signals that only their owners can detect. A frequency calibrated to trigger rapid movement toward any line forming...

Our Journey to Avignon

🇫🇷 Lost in Transition: Our Journey to Avignon (Part 1) When everything that can go wrong—does—sometimes grace still finds you. Our Monday morning trip began on the quiet island of Noirmoutier , where salt marshes and sea breezes whisper of simpler days. From there, our early morning drive was uneventful; we arrived at the Nantes station with plenty of time to spare. From Nantes to Paris Montparnasse, everything went smoothly—so we thought, until it didn't. That's when things started to unravel. If you've never traveled the Paris Metro , imagine a vast underground maze pulsing beneath the city—corridors twisting into one another, trains roaring in and out of the dark, staircases that rise and fall like riddles. It's efficient, yes—but only if you know where you're going. We had over an hour and a half to make our next train to Avignon —plenty of time. Or so we believed. We needed to reach the Gare de Lyon station, where our TGV (high-speed train) was headed south...

On The Fly

Search This Blog