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Gemstone's Refractive Index

Refracive index, the abbreviated is RI, is one of the most important identified of gemstone. Because almost gemstones will have different refractive index, but not all. So if you have to make sure what stone you test, you have to used other method with also. Whatever the refractive index testing have to be the one method you can not deny.

When light passes from one density or type of material (air) into another (the gemstone), it is bent or "refracted." The amount, or degree, that the beam of light is bent will be determined by the density difference between the air and the gem. The measurement used to quantify the amount that a light beam is bent in a given material is known as its "refractive index".

There are two factors when calculating a gem's refractive index: "angle of incident" and "refractive angle." The "incident angle" is the angle of the approaching light as it intersects with the stone's exterior surface. The "refractive angle" is the altered angle of the light as it passes through the stone's interior. The Refractive Index is the ratio of difference between these two angles. Each material has its own unique density and Refractive Index.

The color of a gemstone, and the frequency (color) of the light traveling through it, can also effect its refractive index. As the light's frequency changes, so does its angle of refraction. Higher light frequencies travel through the stone more slowly, while lower frequencies travel faster, causing a spreading effect known as dispersion. As each frequency is reflected it is dispersed throughout the stone's interior at varying speeds and directions. As the scattering and dispersion of light within the stone is increased, so to is the amount of "fire" that is returned to the viewer.

The color of a gemstone, and the frequency (color) of the light traveling through it, can also effect its refractive index. As the light's frequency changes, so does its angle of refraction. Higher light frequencies travel through the stone more slowly, while lower frequencies travel faster, causing a spreading effect known as dispersion. As each frequency is reflected it is dispersed throughout the stone's interior at varying speeds and directions. As the scattering and dispersion of light within the stone is increased, so to is the amount of "fire" that is returned to the viewer.

The table below is the list gemstone's RI that arranged from the gemstone that have the lowest refractive index to highest refractive index.

Gemstone

Refractive index

Double Refraction

Birefringence

Opal

1.37 – 1.52

None

N/A

Fluorite

1.434

None

N/A

Obsidian

1.45 – 1.55

None

N/A

Chrysocolla

1.469 – 1.57

Yes

0.023 -  0.040

Moldavite

1.48 – 1.54

None

N/A

Sodalite

1.48

None

N/A

Hackmanite

1.48

None

N/A

Calcite

1.486 – 1.658

Yes

0.172

Coral

1.486 – 1.658

Yes

White and red : 0.160 – 0.172

Lapis lazuli

1.5

None

N/A

Moonstone

1.518 – 1.526

Yes

0.008

Orthoclase

1.518 – 1.53

Yes

0.008

Ammolite

1.52 – 1.68

Yes

0.155

Pearl

1.52 – 1.69

Yes

0.156

Maw sit sit

1.52 – 1.74

Yes

0.015 – 0.02

Amazonite

1.522 – 1.530

Yes

0.008

Lepidolite

1.525 – 1.586

Yes

0.029 – 0.038

Sunstone

1.525 – 1.548

Yes

0.010

Agate

1.53 – 1.54

Yes

0.001 – 0.004

Bloodstone

1.53 – 1.54

Yes

0.001 – 0.004

Carnelian

1.53 – 1.54

Yes

0.001 – 0.004

Quartz ex. Rose, Smoky and Lemon

1.53 – 1.54

Yes

0.009

Fossil coral

1.53 – 1.54

Yes

0.001 – 0.004

Chrysophase

1.53 – 1.54

Yes

0.004 – 0.009

Chalcedony

1.53 – 1.54

Yes

0.001 – 0.004

Tiger’s eye

1.534 – 1.540

None

N/A

Quartz cat’s eye

1.534 – 1.540

None

N/A

Amber

1.539 – 1.545

None

N/A

Scapolite

1.540 – 1.579

Yes

0.006 – 0.037

Peanut wood

1.54

None

N/A

Jasper

1.54

None

N/A

Iolite

1.542 – 1.578

Yes

0.008 – 0.012

Aventurine

1.53 – 1.54

Yes

0.009

Pietersite

1.544 – 1.553

Yes

0.009

Ametrine

1.544 – 1.553

Yes

0.009

Mystic quartz

1.544 – 1.553

Yes

0.009

Onyx

1.544 – 1.553

Yes

0.001 – 0.004

Rutile quartz

1.544 – 1.553

Yes

0.009

Amethyst

1.544 – 1.553

Yes

0.009

Citrine

1.544 – 1.553

Yes

0.009

Charoite

1.550 – 1.561

Yes

0.004 – 0.009

Andesine labradorite

1.551 – 1.560

Yes

0.008

Verdite

1.552 – 1.576

None

N/A

Hambergite

1.553 – 1.628

Yes

0.007

Labradorite

1.559 – 1.570

Yes

0.008 – 0.010

Serpentine

1.560 – 1.710

Yes

0.008 – 0.014

Morganite

1.562 – 1.602

Yes

0.004 – 0.010

Beryl

1.562 – 1.620

Yes

0.004 – 0.010

Goshenite

1.562 – 1.620

Yes

0.004 – 0.010

Veriscite

1.563 – 1.594

Yes

0.031

Aquamarine

1.564 – 1.596

Yes

0.004 – 0.005

Emerald

1.565 – 1.602

Yes

0.006

Seraphinite

1.576 – 1.599

Yes

0.005 – 0.011

Howlite

1.586 – 1.605

Yes

0.019

Larimar

1.595 – 1.645

Yes

0.038

Rhodochrosite

1.6 – 1.82

Yes

0.208 – 0.220

Nephrite jade

1.600 – 1.627

Yes

0.027

Jadeite

1.600 – 1.627

Yes

0.027

Sugilite

1.607 – 1.611

Yes

0.001 – 0.004

Topaz

1.609 – 1.643

Yes

0.008 – 0.016

Gaspeite

1.61 – 1.81

Yes

0.22

Turquoise

1.61 – 1.65

Yes

0.04

Prehnite

1.611 – 1.699

Yes

0.021 – 0.039

Hemimorphite

1.614 – 1.636

Yes

0.022

Tourmaline ex. Chrome, cat’s eye, rubellite, Paraiba and indicolite

1.614 – 1.666

Yes

0.014 – 0.032

Smitsonite

1.621 – 1.849

Yes

0.228

Andalusite

1.627 – 1.649

Yes

0.007 – 0.013

Apatite

1.628 – 1.649

Yes

0.002 – 0.006

Clinohumite

1.629 – 1.674

Yes

0.029 – 0.045

Danburite

1.630 – 1.636

Yes

0.006 – 0.008

Peridot

1.650 – 1.703

Yes

0.036 – 0.038

Malachite

1.655 – 1.909

Yes

0.254

Silimanite

1.655 – 1.684

Yes

0.014 – 0.021

Axinite

1.656 – 1.704

Yes

0.010 – 0.012

Spodumene ex. Kunzite, Hiddenite

1.660 – 1.681

Yes

0.014 – 0.016

Diopside ex. Chrome, star and Tashmarine

1.664 – 1.730

Yes

0.024 – 0.031

Dumotierite quartz

1.678 – 1.689

Yes

0.015 – 0.037

Ruby - Zoisite

1.691 – 1.700

Yes

0.009

Tanzanite

1.691 – 1.700

Yes

0.009

Idocrase

1.700 – 1.732

Yes

0.002 – 0.012

Diaspore or refer to Zultanite

1.702 – 1.750

Yes

0.048

Kyanite

1.710 – 1.734

Yes

0.015 – 0.033

Spinel

1.712 – 1.762

Rhodonite

1.716 – 1.752

Yes

0.010 – 0.014

Azurite

1.720 – 1.848

Yes

0.108 – 0.110

Garnet ex. Pyrote, Star, Rhodolite, Color change and Hessonite

1.720 – 1.756

None

N/A

Grossular garnet ex. Tsavorite, Grossularite and Mali

1.734 – 1.759

None

N/A

Chrysoberyl

1.746 – 1.763

Yes

0.004 – 0.010

Alexandrite

1.746 – 1.763

Yes

0.004 – 0.010

Corundum ex. Sapphire, Ruby

1.762 – 1.778

Yes

0.008

Garnet ex. Almandite and Spessartite

1.77 – 1.82

None

N/A

Zircon

1.810 -2.024

Yes

0.002 – 0.059

Sphene

1.843 – 2.110

Yes

0.100 – 0.192

Melanite

1.88 – 1.94

None

N/A

Andradrite garnet ex. Demantoid and Topazolite

1.88 – 1.94

None

N/A

Cassiterite

1.997 – 2.098

Yes

0.096 – 0.098

Sphalerite

2.368 – 2.371

None

N/A

Diamond

2.417 – 2.419

None

N/A

Hematite

2.94 – 3.22

Yes

0.287