Color Science of Gem Stones

Color Science of Gem Stones

Key Terms

  • Iridescence Orient
  • Play-of-color Labradorescence
  • Chatoyancy (“cat’s-eye”) Asterism
  • Adularescence
  • Aventurescence
  • Change-of- color (“Alexandrite effect”)
  • Pearlescence
  • Opalescence

Causes of Color in Gemstones

Sourcez: AN UPDATE ON COLOR IN GEMS. PART 1: INTRODUCTION AND COLORS CAUSED BY DISPERSED METAL IONS

Three most common causes of color in gem materials:

  • Dispersed metal ions
  • Charge transfers and other processes that involve multiple ions, and colorcenters.
  • Coloration that are less often seen in gems, such as those that result from physical phenomena (asin opal) or from semiconductor-like properties (as in natural blue diamond).

Source: http://www.scifun.org/chemweek/ColorOfGemstones2017.pdf

THE COLORS OF GEMSTONES

The most common cause of color in gemstones is the presence of a small amount of a transition metal ion. These transition metal ions have an incomplete set of 3electrons. Changes in the energy of these electrons correspond to the energy of visible light. When white light passes through a colored gemstone or is reflected by it, some of the energy of the visible light is absorbed, causing 3electrons in the transition metal ion to undergo an energy change. The light that is transmitted or reflected appears colored, because those colors corresponding to 3d– electron energy transitions have been absorbed. The table lists several common gemstones, their chemical compositions, colors, and the origins of these colors.

A ruby is a crystal of alumina, aluminum oxide, containing a trace of chromium(III) ions replacing some of the aluminum ions. In ruby, each Al3+ ion and Cr3+ ion is surrounded by six oxide ions in an octahedral arrangement.

GemFormulaColorOrigin of color
RubyAl2O3RedCr3+ replacing Al3+ in octahedral sites
EmeraldBe3Al2(SiO3)6page1image48667408 page1image48670688Greenpage1image48674080Cr3+ replacing Al3+ in octahedral site
AlexandriteAl2BeO4page1image48684352 page1image48679952Red/Greenpage1image46942720Cr3+ replacing Al3+ in octahedral site
GarnetMg3Al2(SiO4)3page1image46978464 page1image46979040Redpage1image46980384Fe2+ replacing Mg2+ in 8- coordinate site
page1image47140576 page1image47141088PeridotMg2SiO4Yellow-greenFe2+ replacing Mg2+ in 6- coordinate site
page1image47154784 page1image47155296T ourmalinepage1image47156576 page1image47157088Na3Li3Al6(BO3)3(SiO3)6F4PinkMn2+ replacing Li+ and Al3+ in octahedral site
TurquoiseAl6(PO4)4(OH)84H2OBlue-greenCu2+ coordinated to 4 OH and 2 H2O
Sapphirepage1image88097760 page1image88100240Al2O3BlueIntervalence transition between Fe2+ and Ti4+ replacing Al3+ in adjacent octahedral sites

This arrangement splits the five 3orbitals of Cr3+ into two sets, the dxy, dxz, dyz orbitals and the dx2-y2 and dz2 orbitals. These two sets have different energies. The energy difference between these sets corresponds to the energy of visible light. When white light strikes a ruby, the gem absorbs the light of energy corresponding to the transition of an electron from the lower-energy set of 3orbitals to the higher-energy set. The ruby reflects or transmits the remainder of the light. Because this light is deficient in some energies (those that were absorbed), the light appears colored.

The origin of the color of emeralds is similar to that of the color of rubies. However, the bulk of an emerald crystal is composed of beryl, beryllium aluminum silicate, instead of the alumina which forms rubies. The color is produced by chromium(III) ions, which replace some of the aluminum ions in the crystal. In emeralds, the Cr3+ is surrounded by six silicate ions, rather than the six oxide ions in ruby. These silicate ions also split the 3orbitals of Cr3+ into two sets. However, the magnitude of the energy difference between the sets is different from that produced by the oxide ions in ruby. Therefore, the color of emeralds is different from that of ruby.

Chromium(III) also produces color in alexandrite. The color of this gem is very unusual, because in bright sunlight it appears green, but in incandescent light it appears red. This unusual behavior is a result of the way human vision works. Our eyes are most sensitive to green light. Alexandrite reflects both green and red light. In bright sunlight, the proportion of green light is greater than it is in the light from an incandescent lamp. The light reflected by alexandrite in bright sunlight is rich in green light, to which our eyes are most sensitive, and we perceive the gem as green. The light reflected by alexandrite in incandescent light is much richer in red, and we see the stone as red under these conditions.

Energy transition of the 3orbitals of other transition metal ions are responsible for the colors of other gemstones. Iron(II) produces the red of garnets and the yellow-green of peridots. Manganese(II) is responsible for the pink coloration of tourmaline, and copper(II) colors turquoise.

In some gemstones, the color is caused not by energy changes in a single transition metal ion, but by the exchange of electrons between two adjacent transition metal ions of differing oxidation states. The energy needed to transfer an electron from one ion to another corresponds to the energy of visible light. An example is sapphire. The bulk of sapphire is alumina, as in rubies, but some adjacent pairs of Al3+ ions are replaced by an Fe2+ ion and a Ti4+. When light of the appropriate energy strikes the crystal, energy is absorbed, and an electron moves from the Fe2+ to the Ti4+. Such a movement is called an intervalence transition. An intervalence transition is also responsible for the blue color of aquamarine. In aquamarine, adjacent Al3+ ions in beryl are replaced by an Fe2+ ion and an Fe3+ ion.

Not all gem colors are produced by transition metal ions. In some gemstones, the colors are produced by the presence of foreign atoms with a different number of valence electrons than the ones they replace. These foreign atoms are called color centers. Because the replacement atoms have the wrong number of valence electrons, they can supply or receive an electron from another atom by an intervalence transition. These color centers are often produced by nuclear transformation. An example of such a transformation is the change of a radioactive carbon- 14 atom in diamond into a nitrogen atom through beta particle emission. This leaves an atom of nitrogen in place of the original carbon atom. The nitrogen atom has one more valence electron than the carbon atom. These nitrogen atoms are the cause of the coloration of blue and yellow diamonds. Color centers can be caused artificially as well, by irradiating the gem in a nuclear reactor. Many bright blue and bright yellow diamonds are produced artificially in this manner.

REFERENCES

Chemistry in Britain, 1983, page 1004.
Gems and Gemology, Volume 17, 1981, page 37. Scientific American, October 1980, page 124.

Precious Stones

  • The Diamond
  • The Pearl
  • The Ruby
  • The Sapphire
  • The Emerald
  • The Oriental Cateye
  • The Alexandrite

RGB Colors of Gemstones

Blue Sapphire

Emerald

Ruby

Pearl

Tahitian Cultured Pearls

Diamond

Chrysoberyl (Oriental Cat’s Eye)

Alexandrite

Change in Color due to change in Illuminant

Semi Precious stones

  • The Amethyst
  • The Topaz
  • The Tourmaline
  • The Aquamarine
  • The Chrysoprase
  • The Peridot
  • The Opal
  • The Zircon
  • The Jade
  • The Garnet
  • The Lapis lazuli
  • The Moonstone
  • The Spinel
  • The Turquoise
  • The Agate
  • The Coral
  • The Citrine
  • The Onyx
  • The Chrysolite
  • The Amber
  • The Chrysoberyl
  • The Chalcedony
  • The Morganite
  • The Quartz
  • The Tanzanite

Amethyst

Topaz

London Blue Topaz

Blue Topaz

Tourmaline

The Aquamarine

Chrysoprase

The Peridot

The Opal

The Zircon

The Jade

Garnet

Lapis lazuli

The MoonStone

White Moonstone

Grey Moonstone

The Spinel

Turquoise

Agate

Red Agate

Citrine

Onyx Black

 Chalcedony

Rose Quartz

Color Chemistry of Gemstones

Healing Power of Gemstones and Crystals

Precious Stones and Semi Precious Stones arranged by Color

Precious and Semi Precious Stones and their characteristics

Birthstones by Month

Source: AN UPDATE ON COLOR IN GEMS. PART 3: COLORS CAUSED BY BAND GAPS AND PHYSICAL PHENOMENA

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Optics of Metallic and Pearlescent Colors

Color Change: In Biology and Smart Pigments Technology

Color and Imaging in Digital Video and Cinema

Digital Color and Imaging

On Luminescence: Fluorescence, Phosphorescence, and Bioluminescence

On Light, Vision, Appearance, Color and Imaging

Key Sources of Research

COLOR IN GEMS: THE NEW TECHNOLOGIES

By George R. Rossman

https://www.semanticscholar.org/paper/Color-in-Gems%3A-The-New-Technologies-Rossman/6202b8b7c6bf5db326a4f173813f0e7bd4943c69

A Primer of Gemstones

Nova

https://www.pbs.org/wgbh/nova/article/gemstone-primer/

THE COLORS OF GEMSTONES

Click to access ColorOfGemstones2017.pdf

An UPDATE ON COLOR IN GEMS. PART 1: INTRODUCTION AND COLORS CAUSED BY DISPERSED METAL IONS

By Emmanuel Fritsch and George R. Rossman

AN UPDATE ON COLOR IN GEMS. PART 2: COLORS INVOLVING MULTIPLE
ATOMS AND COLOR CENTERS

By Emmunuel Fritsch and George R. Rossinun

AN UPDATE ON COLOR IN GEMS. PART 3: COLORS CAUSED BY BAND GAPS AND
PHYSICAL PHENOMENA

By Emmanuel Fritsch and George R. Rossman

What Causes the Colour of Gemstones?

What Causes the Colour of Gemstones?

Concerning Precious Stones and Jewels

Issued by Theodore A. Kohn & Son
Jewellers, New York

Palagems

http://www.palagems.com/concerning-precious-stones

7 Gemstone Legends That Will Blow Your Mind

Angara

GEOSC 110H: The Science of Gemstones

Penn State

Gemstones

LEE ANDREW GROAT

https://www.americanscientist.org/article/gemstones

Source of many Images

https://www.leibish.com/rings-jewelry/mozambique-no-heat-pigeon-blood-ruby-three-stone-ring-28510

The origins of color in minerals

KURT NASSAU

Bell Laboratories

Murray Hill, New Jersey 07974

American Mineralogist

Volume 63, pages 219-229, 1978

http://www.minsocam.org/MSA/collectors_corner/arc/color.htm

THE EARLY HISTORY OF GEMSTONE TREATMENTS

By Kurt Nassau

A QUICK GUIDE TO PEARL COLORS

DNA Fingerprinting of Pearls to Determine Their Origins

DOI: 10.1371/journal.pone.0075606

https://www.researchgate.net/publication/257840043_DNA_Fingerprinting_of_Pearls_to_Determine_Their_Origins

New developments in cultured pearl production: use of organic and baroque shell nuclei


January 2013
Authors: Laurent E Cartier University of Lausanne
Michael S. Krzemnicki at University of Basel

https://www.researchgate.net/publication/276269725_New_developments_in_cultured_pearl_production_use_of_organic_and_baroque_shell_nuclei

Blue Nile

https://www.bluenile.com/

Alexandrite Effect: Gemstones That Change Color in Different Light

http://www.geologyin.com/2017/03/alexandrite-effect-not-all-white-light.html

What is Chrysoprase?

http://geologylearn.blogspot.com/2016/12/chrysoprase-gemstone.html

10 World Famous Gemstones

PUBLISHED FRI, JUL 11 200810:07 AM EDTUPDATED WED, JAN 29 20143:11 PM EST

Jessica Mark

https://www.cnbc.com/2008/07/11/10-World-Famous-Gemstones.html

The Causes of Color

Kurt Nassau

Gem Diamonds: Causes of Colors

Hiroshi Kitawaki

Gemmological Association of All Japan, Ueno 5-25-11, Taito-ku, Tokyo 110-0005, Japan (Received 9 May 2007; accepted 1 August 2007)

New Diamond and Frontier Carbon Technology

Vol. 17, No. 3 2007 MYU Tokyo

NDFCT536_full.pdf

Causes of Color in Minerals and Gemstones 

Paul F. Hlava, Sandia National Laboratories pfhlava@sandia.gov


Author: Mayank Chaturvedi

You can contact me using this email mchatur at the rate of AOL.COM. My professional profile is on Linkedin.com.

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