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)

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