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GEMMOLOGY > Colour > Flashcards

Colour Flashcards

1
Q

What is white light?

A

A combination of all the colours in the range of visible light energy. It is made up of a continuous band of wavelengths

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2
Q

What is the visible spectrum?

A

The whole range of colours and wavelengths

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3
Q

What is the range of visible light wavelengths in nm?

A

400nm to 700nm

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4
Q

How is colour produced in gems?

A

During modification of white light, as a result of selective absorption, dispersion and diffraction

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5
Q

What does idiochromatic mean?

A

Gem materials where the colouring element forms an essential part of the atomic structureand are said to be ‘self coloured’

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6
Q

What does Allochromatic mean?

A

Gem materials where the colouring elements are not essential but are present as an impurity, are ‘other coloured’

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7
Q

What colour do the following elements produce?

  1. Titanium
  2. Vanadium
  3. Chromium
  4. Manganese
  5. Iron
  6. Cobalt
  7. Nickel
  8. Copper
A
  1. Titanium - BLUE
  2. Vanadium - GREEN/PINK
  3. Chromium RED/GREEN
  4. Manganese - PINK
  5. Iron - GREEN/BLUE/RED/YELLOW
  6. Cobalt - BLUE
  7. Nickel - GREEN
  8. Copper - GREEN/BLUE
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8
Q

What elements produce colour in diamonds?

A

Boron and nitrogen

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9
Q

Why does the same element produce different colours in different gem materials?

A

Due to the environment and relationship to oxygen in the crystal lattice the energy levels of electrons will differ causing the the brain to influence the colours differently.

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10
Q

Explain selective absorption of light in relation to body colour of gem stones

A

Coloured gems remove (absorb) some wavelengths from white light. The remaining energy is transmitted to brain as residual light which is then interpreted as a single colour - body colour

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11
Q

What does the spectroscope do?

A

It detects the wavelengths in the spectrum where absorption occurs. It splits the light into its constituent wavelengths using an optical device called diffraction grating.

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12
Q

What is dispersion?

A

The splitting of white light into the spectral colours when it passes through two inclined surfaces of a transparent material. It occurs because each colour is refracted as at a different angle. The amount of dispersion in a gemstone is the difference in refractive index measured in red and blue light

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13
Q

What colour is refracted the most?

A

Violet

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14
Q

What colour is refracted the least?

A

Red

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15
Q

What are the dispersion values of Diamonds, CZs and Synthetic Moissanite?

A

Diamond - 0.044
CZ - 0.065
Synthetic Moissanite - 0.104

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16
Q

What is diffraction?

A

When light passes through a minute slit or hole they become slightly spread out (diffracted). If white light is passed through a very fine grid of parallel line imprinted on a transparent sheet, the diffracted waves produce a spread out spectra. This is called DIFFRACTION GRATING

17
Q

How is colour effect in opals produced?

A

Interference of light is diffracted through gaps between minute spheres in the opal structure.

18
Q

Why do emission lines occur in the spectrum?

A

Result of fluorescence and occurs when shorter wavelength energy is converted into longer wavelength light.

19
Q

What light should NOT be used when using a spectroscope?

A

LED torch
Fluorescent lamp
Daylight

20
Q

What is the Chelsea Colour Filter?

A

It is a colour filter that transmits only deep red and yellow/green light. originally desgined to distinguish between natural and simulant emeralds. however no longer used for this purpose. However they can help distinguish between other similar looking stones

21
Q

Explain the Colour Change Effect

A

Different white light sources produce different combinations of wavelengths. Gemstones can be different colours in different types of incident white light.. EG Alexandrite is red under tungsten light and green in daylight.

22
Q

What is Pleochroism?

A

It is a body colour effect possessed by many optically anisotropic coloured gemstones. Pleochroic materials selectively absorb light waves differently in different vibration directions. A different body colour is transmitted by each vibration direction.

23
Q

Which stones display DICHROISM?

A

Uniaxial materials (tetragonal, trigonal and hexagonal). they have two principle vibration directions which produce a two coloured effect called dichroism.

24
Q

Which stones display TRICHROISM?

A

Biaxial materials (orthorhombic, monoclinic and triclinic). They have three principle vibration directions which exhibit three colours.

25
Q

Why is testing for pleochroism important?

A
  1. Distinguishes between anisotropic materials and isotropic materials.
  2. Can show characteristic results that help in identification.
  3. Knowing where the best colour is shown helps in lapidary.
26
Q

How does the London Dichroscope work?

A

It separates the two vibration directions using two polarising filters at 90 degrees to each other. This allows the two pleochroic colours to be viewed side by side.

27
Q

What light should be used with a dichroscope?

A

Diffused, unpolarised, tungsten light

28
Q

How is iridescence caused?

A

Caused by interference effects in light reflected from thin layers, cracks and thin film surfaces.

GEMMOLOGY (5 decks)

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