Gem_5208 Color of Gemstones
- Visible Range of Electromagnetic Spectrum
- Cause of Color
- Coloring Elements
- Allochromatic and Idiochromatic Gem Minerals
- Physical Optics – Dispersion and Diffraction
- The Selective absorption of gem minerals
- Visible Spectrum of Gem Minerals
- Construction, uses and typically results of Specreoscope
- Pleochroism
- Construction, uses and typically results of Dichroscope
- Color Filters
- Construction, uses and typically results of Chelsia Color Filter
Gem_5208 Color of Gemstones
1.1 Visible Range of Electromagnetic Spectrum
Introduction to the Electromagnetic Spectrum
The electromagnetic spectrum encompasses all types of electromagnetic radiation, from gamma rays to radio waves.
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye, ranging from approximately 380 nm (violet) to 750 nm (red).
Explanation of the Visible Light Range
Light within this range can be seen as different colors, each corresponding to a specific wavelength.
Violet: 380450 nm
Blue: 450495 nm
Green: 495570 nm
Yellow: 570590 nm
Orange: 590620 nm
Red: 620750 nm
Significance of the Visible Range in Gemology
Gemstones are often identified and valued based on their color, which is a result of how they interact with visible light.
Diagram of the Electromagnetic Spectrum
A visual representation showing the entire electromagnetic spectrum with the visible light range highlighted.
Applications in Gemology
Understanding how gemstones absorb, transmit, and reflect different wavelengths of light helps in identifying and assessing their quality.
1.2 Cause of Color
Interaction of Light with Matter
Color in gemstones is primarily due to the interaction between light and the atoms or molecules in the gem.
Absorption and Emission of Light
When light hits a gemstone, certain wavelengths are absorbed while others are transmitted or reflected. The wavelengths that are not absorbed determine the color we perceive.
Some gemstones exhibit fluorescence, where they emit light of a different wavelength when exposed to ultraviolet light.
Examples of Color Causes in Gemstones
Ruby: Red color due to chromium ions.
Emerald: Green color due to chromium or vanadium ions.
Sapphire: Blue color due to iron and titanium ions.
1.3 Coloring Elements
Role of Transition Metals
Transition metals have partially filled dorbitals that can absorb specific wavelengths of light, leading to vibrant colors.
The specific wavelengths absorbed depend on the arrangement of the electrons in the dorbitals and the crystal field around the metal ion.
Common Coloring Elements
Chromium (Cr): Produces red and green colors (e.g., Ruby, Emerald).
Iron (Fe): Can produce yellow, green, blue, and brown colors (e.g., Peridot, Sapphire).
Manganese (Mn): Produces pink and red colors (e.g., Rhodonite, Spessartine Garnet).
Impact of Chemical Composition
The presence and concentration of these elements, as well as their oxidation state, greatly influence the color of the gemstone.
1.4 Allochromatic and Idiochromatic Gem Minerals
Definitions
Allochromatic: Minerals that get their color from impurities or trace elements not part of their essential chemical composition (e.g., Quartz can be amethyst (purple) due to iron impurities).
Idiochromatic: Minerals that derive their color from elements that are an essential part of their chemical composition (e.g., Peridot, which is green due to iron).
Examples and Differences
Allochromatic: Blue Sapphire (iron and titanium impurities), Amethyst (iron impurities).
Idiochromatic: Peridot (iron), Malachite (copper).
Impact on Gem Identification
Knowing whether a gem is allochromatic or idiochromatic helps gemologists determine the origin of its color and identify the gem more accurately.
1.5 Physical Optics – Dispersion and Diffraction
Basic Concepts of Physical Optics
Physical optics studies the wave nature of light and how it interacts with different materials.
Dispersion in Gemstones
Dispersion: The splitting of white light into its constituent colors (like a prism).
In gemstones, dispersion results in “fire,” which is the display of rainbow colors seen in some gems (e.g., Diamond).
Diffraction in Gemstones
Diffraction: The bending of light around the edges of an object or through a slit, which can create spectral colors.
Some gemstones, like opals, show diffraction patterns due to their internal structure.
Practical Examples
Diamond: High dispersion gives it a brilliant “fire.”
Opal: Shows diffraction, resulting in a playofcolor effect.
1.6 The Selective Absorption of Gem Minerals
Mechanism of Selective Absorption
Different wavelengths of light are absorbed based on the electronic structure of the atoms and the crystal field of the gem.
The absorbed wavelengths are missing from the spectrum of transmitted or reflected light, giving the gem its color.
Factors Affecting Absorption
Chemical Composition: The specific elements present in the gem.
Crystal Structure: The arrangement of atoms in the gem.
Impurities: Trace elements that can alter absorption.
Examples in Gemstones
Emerald: Absorbs light in the red and blue regions, reflecting green.
Amethyst: Absorbs light in the yellow and green regions, reflecting purple.
1.7 Visible Spectrum of Gem Minerals
Visible Spectrum Analysis
Analyzing the spectrum of light absorbed and transmitted by a gemstone helps identify its color and possible impurities.
Spectral Patterns of Common Gemstones
Ruby: Sharp absorption lines in the green and blue regions.
Sapphire: Broad absorption bands due to iron and titanium.
Application in Gem Identification
By comparing the visible spectrum of a gem with known standards, gemologists can identify the gem and detect treatments or enhancements.
1.8 Construction, Uses, and Typical Results of Spectroscope
Spectroscope Construction
Components: Slit, collimating lens, prism or diffraction grating, and an eyepiece.
Types: Prism spectroscope and diffraction grating spectroscope.
Uses of Spectroscope
Used to observe the absorption spectrum of gemstones.
Helps identify the presence of specific elements and diagnose treatments.
Interpreting Spectroscope Results
Ruby: Sharp absorption lines at 692.8 nm and 694.2 nm (chromium).
Sapphire: Broad bands due to iron and titanium.
1.9 Pleochroism
Definition and Explanation
Pleochroism is the property of a gemstone to exhibit different colors when viewed from different angles, due to anisotropy in the crystal structure.
Types of Pleochroism
Dichroism: Two different colors (e.g., Tourmaline).
Trichroism: Three different colors (e.g., Tanzanite).
Examples of Pleochroic Gemstones
Tanzanite: Blue, violet, and burgundy.
Andalusite: Green, brown, and red.
Applications in Gemology
Used to identify gemstones and understand their internal structure.
1.10 Construction, Uses, and Typical Results of Dichroscope
Dichroscope Construction
Components: A calcite or polarizing filter, an eyepiece, and a viewing tube.
Uses of Dichroscope
Detects pleochroism in gemstones by showing two colors side by side.
Interpreting Dichroscope Results
Tourmaline: Shows two distinct colors.
Tanzanite: Shows three colors, depending on the angle.
1.11 Color Filters
Introduction to Color Filters
Color filters selectively transmit light of certain wavelengths while blocking others.
Types of Color Filters
Chelsea Filter: Differentiates between natural and synthetic emeralds.
Cobalt Filter: Used to identify cobalttreated stones.
Practical Examples
Emerald: Appears red under the Chelsea filter if natural.
Ruby: Appears deep red under a red filter, helping distinguish it from spinel.
1.12 Construction, Uses, and Typical Results of Chelsea Color Filter
Chelsea Filter Construction
Components: Made of a combination of cobalt and rare earth elements, designed to transmit red and green light.
Uses of Chelsea Color Filter
Helps distinguish between natural emeralds and imitations or synthetics.
Useful in identifying chromiumcontaining gems.
Interpreting Chelsea Filter Results
Natural Emerald: Appears red or pink.
Synthetic Emerald: Appears green or bluishgreen.