chemical-properties.yaml

title: Chemical Properties
description: Gem chemistry, chromophores and colour causes, allochromatic vs idiochromatic minerals, charge transfer, and colour centres.
order: 3
category: fundamentals
difficulty: intermediate
icon: flask
related:
  - fundamentals/optical-properties
  - fundamentals/crystal-systems
  - identification/treatments
tags:
  - chemistry
  - chromophores
  - colour
  - formulas
  - composition

sections:
  - title: Introduction
    content: |
      Understanding gem chemistry explains why gems have their colours, what makes
      one variety of a species different from another, and how treatments work.
      Chemical composition determines many physical and optical properties.

      The study of chromophores (colour-causing agents) is particularly important
      for gem identification, treatment detection, and understanding colour stability.

  - title: Allochromatic vs Idiochromatic Minerals
    content: |
      Minerals are classified by how they obtain their colour.
    comparison:
      items:
        - title: Allochromatic (Other-Coloured)
          points:
            - Colour from trace impurities
            - Pure form is colourless
            - Same mineral can be many colours
            - Impurity not part of ideal formula
            - "Examples: corundum, beryl, quartz"
        - title: Idiochromatic (Self-Coloured)
          points:
            - Colour from essential elements
            - Colour intrinsic to formula
            - Always the same general colour
            - Colour-causing element required
            - "Examples: peridot (Fe), malachite (Cu)"

  - title: Allochromatic and Idiochromatic Examples
    table:
      caption: Colour Origin Classification
      headers:
        - Mineral
        - Classification
        - Colour Cause
        - Resulting Colours
      rows:
        - ["Corundum", "Allochromatic", "Pure Al₂O₃ is colourless", "Ruby (Cr), sapphire (Fe/Ti)"]
        - ["Beryl", "Allochromatic", "Pure Be₃Al₂Si₆O₁₈ is colourless", "Emerald (Cr), aquamarine (Fe)"]
        - ["Quartz", "Allochromatic", "Pure SiO₂ is colourless", "Amethyst (Fe), citrine (Fe)"]
        - ["Tourmaline", "Allochromatic", "Complex borosilicate", "Many colours (various)"]
        - ["Peridot", "Idiochromatic", "Fe in formula (Mg,Fe)₂SiO₄", "Always green"]
        - ["Malachite", "Idiochromatic", "Cu in formula Cu₂CO₃(OH)₂", "Always green"]
        - ["Turquoise", "Idiochromatic", "Cu in formula", "Always blue-green"]

  - title: Transition Metal Chromophores
    content: |
      Transition metals are the primary colour-causing agents in gems. Their partially
      filled d-orbitals can absorb specific wavelengths of light.
    subsections:
      - title: Chromium (Cr³⁺)
        content: |
          Chromium is responsible for some of the most prized gem colours:

          - **Ruby**: Red corundum (Cr in Al₂O₃)
          - **Emerald**: Green beryl (Cr in Be₃Al₂Si₆O₁₈)
          - **Alexandrite**: Colour-change chrysoberyl
          - **Red spinel**: Cr-coloured MgAl₂O₄
          - **Chrome tourmaline**: Green tourmaline
          - **Tsavorite**: Green grossular garnet

          The colour Cr³⁺ imparts depends on the crystal-field environment: shorter
          Cr–O bond distances and stronger crystal-field splitting (as in ruby, Cr in
          corundum lattice) shift absorption toward red transmission; longer bonds and
          weaker fields (as in emerald, Cr in beryl lattice) shift toward green
          transmission. The simple oxide-vs-silicate rule fails — red spinel is an
          oxide, yet the absorption pattern matches the field environment, not the
          anion class. (Source: Gem-A D7-24.)

      - title: Iron (Fe²⁺ and Fe³⁺)
        content: |
          Iron is the most common colouring agent in gems:

          - **Fe²⁺**: Blue-green colours (aquamarine, blue tourmaline)
          - **Fe³⁺**: Yellow-brown colours (citrine, yellow sapphire)
          - **Fe²⁺/Fe³⁺ combination**: Can produce various colours

          Iron content often affects other properties:
          - Quenches fluorescence (Thai ruby vs Burmese ruby)
          - Affects response to heat treatment
          - Can be modified by HPHT treatment

      - title: Other Transition Metals
        table:
          headers:
            - Element
            - Ion
            - Typical Colour
            - Example Gems
          rows:
            - ["Vanadium", "V³⁺", "Green or colour-change", "Tsavorite, colour-change sapphire"]
            - ["Manganese", "Mn²⁺", "Pink to orange", "Rhodonite, spessartine, kunzite"]
            - ["Manganese", "Mn³⁺", "Purple to red", "Piemontite"]
            - ["Copper", "Cu²⁺", "Blue to green", "Turquoise, malachite, Paraíba"]
            - ["Cobalt", "Co²⁺", "Blue", "Cobalt blue spinel, blue glass"]
            - ["Titanium", "Ti³⁺/Ti⁴⁺", "Blue (with Fe)", "Blue sapphire (charge transfer)"]
            - ["Nickel", "Ni²⁺", "Green", "Chrysoprase"]

  - title: Chromophore Colour Chart
    table:
      caption: Major Chromophores and Their Colours
      headers:
        - Chromophore
        - Colour
        - Host Minerals
        - Example Gems
      rows:
        - ["Cr³⁺", "Red", "Oxides (corundum, spinel)", "Ruby, red spinel"]
        - ["Cr³⁺", "Green", "Silicates (beryl, garnet)", "Emerald, tsavorite"]
        - ["Fe²⁺", "Blue-green", "Various", "Aquamarine, blue tourmaline"]
        - ["Fe³⁺", "Yellow-brown", "Various", "Citrine, yellow sapphire"]
        - ["Fe²⁺-Ti⁴⁺", "Blue", "Corundum", "Blue sapphire"]
        - ["Cu²⁺", "Blue-green", "Phosphates, carbonates", "Turquoise, Paraíba tourmaline"]
        - ["Mn²⁺", "Pink-orange", "Silicates", "Kunzite, morganite, rhodonite"]
        - ["V³⁺", "Green", "Beryl, grossular", "Some emerald, tsavorite"]
        - ["Co²⁺", "Blue", "Spinel, glass", "Cobalt spinel"]

  - title: Charge Transfer Mechanisms
    content: |
      Some colours result from electron transfer between adjacent ions rather than
      absorption by a single ion.
    subsections:
      - title: Fe²⁺ → Ti⁴⁺ Intervalence Charge Transfer
        content: |
          The classic example of charge transfer colours blue sapphire:

          - Electron transfers from Fe²⁺ to Ti⁴⁺
          - Absorbs red/yellow light → appears blue
          - Requires both iron and titanium present
          - Neither alone would produce blue

          This mechanism explains why blue sapphire shows characteristic three-band
          absorption spectrum at 450, 460, 470 nm.

      - title: Fe²⁺ → Fe³⁺ Intervalence Charge Transfer
        content: |
          Iron-iron charge transfer affects colour in several gems:

          - Contributes to blue in some sapphires
          - Affects colour in aquamarine
          - Can create greenish-blue tints

      - title: Other Charge Transfer
        content: |
          - **O²⁻ → Fe³⁺**: Contributes to yellow in some minerals
          - **O²⁻ → Cr⁶⁺**: Yellow (chromate compounds)

          Charge transfer absorptions are typically broad bands rather than sharp
          lines in the absorption spectrum.

  - title: Colour Centres
    content: |
      Colour centres are crystal defects that can absorb light. They can be created
      or destroyed by radiation and heating.
    subsections:
      - title: Types of Colour Centres
        content: |
          - **F-centres**: Electron trapped at anion vacancy (Farbzentrum = colour centre)
          - **H-centres**: Hole trapped at cation site
          - **Electron-hole pairs**: Various defect combinations

          Colour centres are responsible for:
          - Smoky quartz (irradiated rock crystal)
          - Blue topaz (irradiated colourless topaz)
          - Some fancy colour diamonds

      - title: Radiation-Induced Colour
        table:
          headers:
            - Starting Material
            - Treatment
            - Result
            - Stability
          rows:
            - ["Colourless quartz", "Gamma irradiation", "Smoky quartz", "Stable"]
            - ["Colourless topaz", "Irradiation + heat", "Blue topaz", "Stable"]
            - ["Colourless beryl", "Irradiation", "Yellow (Maxixe)", "Fades in light"]
            - ["Diamond", "Irradiation", "Green/blue surface", "Permanent"]
            - ["Pearl", "Gamma irradiation", "Grey/black", "Stable"]

      - title: Colour Centre Stability
        content: |
          Colour centre stability varies:

          - **Stable**: Smoky quartz, treated blue topaz
          - **Unstable**: Maxixe beryl (fades in light), some kunzite
          - **Heat-sensitive**: Many can be bleached by heating

          Understanding stability is important for predicting colour permanence.

  - title: Diagnostic Absorption Spectra
    content: |
      The spectroscope can identify materials by their characteristic absorption patterns.
      Not all coloured materials have recognizable spectra, and some colourless materials
      (zircon, diamond, jadeite) may still show diagnostic absorption. The following are
      the most diagnostically useful spectra from the Gem-A Diploma course.
    table:
      caption: Key Diagnostic Absorption Spectra
      headers:
        - Material
        - Chromophore
        - Key Wavelengths
        - Diagnostic Notes
      rows:
        - ["Chrysoberyl", "Fe", "Band centred at 444 nm", "Distinguishes yellow/greenish/brown chrysoberyl from sapphire; useful for cat's eyes in settings"]
        - ["Alexandrite", "Cr", "Lines in red; broad band in yellow-green", "Pleochroic variation visible — spectrum shifts between daylight and tungsten light"]
        - ["Synthetic colour-change sapphire", "V + Cr", "Sharp fine line at 475 nm", "Diagnostic for vanadium in corundum; line may be faint"]
        - ["Jadeite (pale specimens)", "Fe", "Fine line at 437 nm", "Seen in pale specimens of various colours; green stones may also show Cr lines in red"]
        - ["Dyed jadeite", "Fe + Cr-dye", "Weak band(s) in red + 437 nm line", "Chromium-green dye gives additional band(s) in red; diagnostic for dye detection"]
        - ["Diamond (Cape yellow)", "N", "Fine line at 415 nm", "Most prominent of several fine lines; easier to see when diamond is cooled (liquid nitrogen or dry ice)"]
        - ["Apatite", "REE", "Multiple fine-line bands in yellow-green", "'Didymium' or rare earth spectrum; each band consists of many fine lines ending in a sharp edge"]
        - ["Pink YAG", "REE", "Cluster of lines/bands in yellow-orange", "Example of REE-doped artificial material; CZ and glass may show similar spectra"]

  - title: Chemical Formulas Reference
    content: |
      Chemical formulas help understand gem composition and elemental substitutions.
    subsections:
      - title: Major Gem Species
        table:
          headers:
            - Gem
            - Mineral
            - Formula
          rows:
            - ["Diamond", "Diamond", "C"]
            - ["Ruby/Sapphire", "Corundum", "Al₂O₃"]
            - ["Emerald/Aquamarine", "Beryl", "Be₃Al₂Si₆O₁₈"]
            - ["Alexandrite/Cat's eye", "Chrysoberyl", "BeAl₂O₄"]
            - ["Spinel", "Spinel", "MgAl₂O₄"]
            - ["Peridot", "Olivine", "(Mg,Fe)₂SiO₄"]
            - ["Tanzanite", "Zoisite", "Ca₂Al₃(SiO₄)₃(OH)"]
            - ["Tourmaline", "Tourmaline", "Complex borosilicate"]
            - ["Topaz", "Topaz", "Al₂SiO₄(F,OH)₂"]
            - ["Quartz", "Quartz", "SiO₂"]
            - ["Zircon", "Zircon", "ZrSiO₄"]

      - title: Garnet Group
        table:
          headers:
            - Variety
            - Species
            - Formula
          rows:
            - ["Pyrope", "Pyrope", "Mg₃Al₂(SiO₄)₃"]
            - ["Almandine", "Almandine", "Fe₃Al₂(SiO₄)₃"]
            - ["Spessartine", "Spessartine", "Mn₃Al₂(SiO₄)₃"]
            - ["Grossular", "Grossular", "Ca₃Al₂(SiO₄)₃"]
            - ["Andradite", "Andradite", "Ca₃Fe₂(SiO₄)₃"]
            - ["Uvarovite", "Uvarovite", "Ca₃Cr₂(SiO₄)₃"]

      - title: Feldspar Group
        table:
          headers:
            - Variety
            - Species
            - Formula
          rows:
            - ["Moonstone", "Orthoclase/Albite", "KAlSi₃O₈ / NaAlSi₃O₈"]
            - ["Labradorite", "Labradorite", "(Ca,Na)(Al,Si)₄O₈"]
            - ["Amazonite", "Microcline", "KAlSi₃O₈"]
            - ["Sunstone", "Oligoclase", "(Na,Ca)(Al,Si)₄O₈"]

  - title: Band Gap Theory
    content: |
      All gem materials are held together by electronic bonding. In some materials,
      electrons can move throughout the crystal structure at a higher energy level than
      those bound within atomic orbitals. The gap between the atom-bound "valence band"
      electrons and the wandering "conduction band" electrons is the electron energy band
      gap — a "forbidden" zone whose width greatly influences optical properties.
    subsections:
      - title: Three Material Types
        content: |
          Materials having a band gap can be grouped into three types:

          1. **Large band gap** (wider than visible light range): Even violet light cannot
             excite electrons across the gap. No visible light is absorbed, so the material
             appears transparent and colourless when pure. Most gemstones fall into this
             category — pure diamond, corundum, quartz, and topaz.

          2. **Small band gap** (narrower than red light energy): All visible light interacts
             with electrons and is absorbed, making the material opaque.

          3. **Overlapping band gap**: The gap energy partially overlaps the visible spectrum,
             causing selective absorption and a residual colour.

      - title: Diamond Band Gap Colour
        content: |
          Diamond provides the key example of band gap colour through impurity energy levels:

          **Nitrogen (yellow)**: Carbon has four valence electrons; nitrogen has five. When
          nitrogen replaces a carbon atom, the extra electron creates a donor level within the
          band gap. Excitation of this electron absorbs light from the middle of the visible
          spectrum through blue and violet, producing a pronounced yellow ("canary") colour.
          This is distinct from the "Cape" yellow caused by nitrogen vacancy colour centres.

          **Boron (blue)**: Boron has only three valence electrons, creating an acceptor level
          near the top of the valence band. Absorption occurs at maximum in the near infrared,
          extending into lower-energy visible light. Higher-energy blue and violet light is
          transmitted, producing blue colour. Because the boron level is close to the valence
          band, electrons can be thermally excited at room temperature, leaving "holes" that
          allow electrical current — boron-containing diamonds conduct electricity.

  - title: Physical Optics Colour
    content: |
      Not all colour in gemstones results from absorption of light by electronic mechanisms.
      Physical phenomena can also produce colour through the interaction of light with
      structures within or on the surface of a material.
    subsections:
      - title: Dispersion
        content: |
          Dispersion separates white light into its component colours ("fire"). The degree
          of dispersion depends on the refractive index variation across wavelengths. Diamond's
          high dispersion (0.044) produces the spectral flashes characteristic of a well-cut
          brilliant.

      - title: Diffraction
        content: |
          The play-of-colour in precious opal is caused by diffraction of light by regularly
          stacked three-dimensional groups of silica spheres. These spheres are only a few
          hundred nanometres in diameter, with gaps of similar size to visible light wavelengths.

          The actual colours seen depend on the size of the spheres:
          - Large spheres: full range of colours from violet to red (most desirable)
          - Medium spheres: violet to green only
          - Small spheres: only violet and blue light can pass between them

      - title: Thin-Film Interference
        content: |
          Thin-film interference colour arises from closely-spaced double reflections from
          thin films, layers of differing composition, or thin-film cavities such as cracks.
          When two reflected rays travel in the same direction and their wave peaks coincide
          ("in phase"), they reinforce each other. When peak meets trough ("out of phase"),
          they cancel — this is interference.

          Examples in gemmology:
          - **Labradorite**: iridescent colours from interference at thin compositional layers
          - **Pearl nacre**: lustre from diffraction at overlapping platy aragonite crystals
            combined with interference from thin nacre layers
          - **Crack iridescence**: colours from thin films of air in cracks (as in topaz,
            glass, or quartz) — similar to oil films on water

      - title: Scattering
        content: |
          Scattering occurs when light is randomly reflected by particles within a substance.
          Blue light is typically much more strongly scattered than red.

          The scattering effect is most noticeable when particles are smaller than approximately
          400 nm in size. This causes the blue adularescence (sheen) in fine-quality moonstone,
          where scattering is caused by submicroscopic particles of albite feldspar.

          As particles get larger, other colours such as red and green may be seen at certain
          angles. Larger particles still produce a whitish effect called opalescence — seen in
          materials such as common opal and milky quartz. This should not be confused with the
          play-of-colour effect in precious opal.

      - title: Inclusions
        content: |
          Fine needle-like inclusions can interact with light to produce optical phenomena:

          - **Chatoyancy** (cat's eye effect): caused by scattering of light from parallel
            needle-like inclusions
          - **Asterism** (star effect): caused by multiple sets of oriented needle-like
            inclusions (typically rutile silk in corundum)

  - title: Trace Element Effects
    content: |
      Minor and trace elements significantly affect gem properties beyond colour.
    subsections:
      - title: Property Modifications
        content: |
          Trace elements can affect:

          - **Fluorescence**: Cr activates; Fe quenches
          - **Hardness**: Some substitutions affect durability
          - **Specific gravity**: Heavier elements increase SG
          - **Refractive index**: Affects RI slightly
          - **Treatment response**: Determines if treatment works

      - title: Origin Indicators
        content: |
          Trace element ratios can indicate geographic origin:

          - **Ga/Fe ratio**: Distinguishes some sapphire origins
          - **Cr/V ratio**: Helps with emerald origin
          - **Cu concentration**: Paraíba tourmaline identification
          - **Li content**: Can distinguish some deposits

          Advanced testing (LA-ICP-MS, LIBS) measures trace elements for origin
          determination.

  - title: Practical Applications
    callout:
      type: tip
      title: Why Chemistry Matters
      text: |
        Understanding gem chemistry helps with:

        1. **Colour prediction**: Know what causes colour
        2. **Treatment understanding**: Why treatments work
        3. **Stability assessment**: Will colour fade?
        4. **Origin clues**: Trace elements indicate source
        5. **Quality factors**: Why some colours are better
        6. **Spectroscopy interpretation**: Connect spectra to chemistry

        Chemistry forms the foundation for advanced gemmological understanding.