Chemical Properties
Gem chemistry, chromophores and colour causes, allochromatic vs idiochromatic minerals, charge transfer, and colour centres.
Introduction
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.
Allochromatic vs Idiochromatic Minerals
Minerals are classified by how they obtain their colour.
Allochromatic (Other-Coloured)
- Colour from trace impurities
- Pure form is colourless
- Same mineral can be many colours
- Impurity not part of ideal formula
- Examples: corundum, beryl, quartz
Idiochromatic (Self-Coloured)
- Colour from essential elements
- Colour intrinsic to formula
- Always the same general colour
- Colour-causing element required
- Examples: peridot (Fe), malachite (Cu)
Allochromatic and Idiochromatic Examples
| Mineral | Classification | Colour Cause | Resulting Colours |
|---|---|---|---|
| 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 |
Transition Metal Chromophores
Transition metals are the primary colour-causing agents in gems. Their partially
filled d-orbitals can absorb specific wavelengths of light.
Chromium (Cr³⁺)
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
Chromium typically produces red in oxide minerals and green in silicates,
though the host mineral structure affects the exact colour.
Iron (Fe²⁺ and Fe³⁺)
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
Other Transition Metals
| Element | Ion | Typical Colour | Example Gems |
|---|---|---|---|
| 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 |
Chromophore Colour Chart
| Chromophore | Colour | Host Minerals | Example Gems |
|---|---|---|---|
| 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 |
Charge Transfer Mechanisms
Some colours result from electron transfer between adjacent ions rather than
absorption by a single ion.
Fe²⁺ → Ti⁴⁺ Intervalence Charge Transfer
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.
Fe²⁺ → Fe³⁺ Intervalence Charge Transfer
Iron-iron charge transfer affects colour in several gems:
- Contributes to blue in some sapphires
- Affects colour in aquamarine
- Can create greenish-blue tints
Other Charge Transfer
- 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.
Colour Centres
Colour centres are crystal defects that can absorb light. They can be created
or destroyed by radiation and heating.
Types of Colour Centres
- 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:
Radiation-Induced Colour
| Starting Material | Treatment | Result | Stability |
|---|---|---|---|
| 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 |
Chemical Formulas Reference
Chemical formulas help understand gem composition and elemental substitutions.
Major Gem Species
| Gem | Mineral | Formula |
|---|---|---|
| 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 |
Garnet Group
| Variety | Species | Formula |
|---|---|---|
| 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 |
Feldspar Group
| Variety | Species | Formula |
|---|---|---|
| Moonstone | Orthoclase/Albite | KAlSi₃O₈ / NaAlSi₃O₈ |
| Labradorite | Labradorite | (Ca,Na)(Al,Si)₄O₈ |
| Amazonite | Microcline | KAlSi₃O₈ |
| Sunstone | Oligoclase | (Na,Ca)(Al,Si)₄O₈ |
Trace Element Effects
Minor and trace elements significantly affect gem properties beyond colour.
Property Modifications
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
Origin Indicators
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.