Chemical Properties

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 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

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.

Iron-iron charge transfer affects colour in several gems:

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

• 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.

• 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

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.

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.

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.

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.

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

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

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.

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)

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

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.