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Refractive Index – Optical Property

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Light Refraction Through A Glass Medium

The Refractive Index (RI) is one of the most important optical property of a gemstone.  Once the RI of a gemstone is known, one can easily identify the gemstone, given another few tests to confirm its identity.

Let us go back to the fundamentals (Read here for fundamentals) to understand what is Refractive Index.

Reflection

Light Reflection

When a light ray strikes a shiny surface part of its light ray is reflected back.  Remember your science lesson that the Law of Reflection states that the angle of incidence is equal to the angle of reflectance and that both these rays are on the same plane.

Refraction

Light Refraction

When a light ray passes from air to another medium, such as a gemstone, its velocity is reduced and the light ray is bent. Refraction is a commonly observed phenomenon in our everyday life.  When a pencil is placed in a glass one will observe that the pencil appears bent. Or you will see a swimming pool is actually shallower than what you actually see because of refraction.

Refractive Index

Now, I will not go into the complex formula of how the RI is calculated using angle of sine and cosine.  Instead, a much more easier approach is to understand that the RI is the following ratio:


Refractive Index = Velocity of Light in Air / Velocity of Light In Medium


Refractive Index can be measured using a Refractometer (which will be cover in another post).

Light Refraction Within A Diamond

The RI of the following gemstones are:

Jadeite Jade     – 1.66

Nephrite Jade   – 1.60

Corundum          – 1.762 – 1.77

Spinel                   –  1.718

Later we will see how a Refractometer can determine the RI of a gemstone, which is one of the most important test for a gemologist in the identification of a gemstone.

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I am a Graduate Gemologist trained at the Gemological Institute of America (GIA) in New York City, USA. I hold an MBA degree from Cranfield University, United Kingdom, and a Bachelor degree in Mathematics. My earlier profession was a banker until I found jade in Myanmar (Burma) in the early 90s. I have traveled to the fabled Hpakan Jade mines, and Mogok, the world’s famous rubies and sapphires mines in upper Burma, with my second son. Three of my children are also Graduate Gemologist, GIA, NYC and they deal in diamonds, gemstones and jade. 我是在美国纽约市的美国宝石学院(GIA)接受过培训的宝石研究学家。 我拥有英国克兰菲尔德大学的工商管理硕士学位和数学学士学位。我以前的职业是银行家,直到90年代初我在缅甸接触到玉石。我曾经和我的次子一起去过缅甸上流传说中的哈帕翡翠矿山和莫谷矿山, 莫谷矿山是世界上著名的红宝石和蓝宝石矿山。我的三个孩子都是纽约市GIA毕业的宝石研究学家, 他们专门处理钻石,宝石和玉石.

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  11. Hello SH, The gems instruments used for gems testing costs around US$1,200, excluding the microscope and spectrometer. China made gems instruments are much cheaper but I am not sure of their reliability. I am a fervent GIA supporter and most of my gems are bought from GIA. You can go to this website and browse, http://www.gia.edu/nav/toolbar/instruments/index.html.

    For gold testing there is a type of Gold Tester costing around $500 or some cheaper.

  12. Arthur,

    Are the tools or equipment needed for testing the RI of jadeite jade and other gems very expensive?

    How does one test if a piece of jewellery is real gold or fake gold?

    Thanks.

    SH

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