Growth Features

Growth features are internal patterns formed during crystallisation that are intrinsic
to a crystal's own development, rather than foreign material enclosed during growth.
They include colour zoning, growth tubes, twinning lamellae, phantom outlines,
internal graining, and strain patterns, each reflecting conditions that shifted while
the crystal formed. Their diagnostic power is considerable: hexagonal angular colour
zoning in corundum is a reliable natural-origin marker, since no synthetic method
replicates the irregular, face-controlled banding that develops over geological time.
Conversely, curved striae in flame-fusion synthetic corundum and chevron (zigzag)
zoning in hydrothermal synthetic emerald are unambiguous indicators of laboratory
origin ^[1]. Zoning patterns also aid provenance:
Sri Lankan blue sapphire typically shows wide hexagonal bands, while Burmese ruby
often displays irregular swirls ^[2]. Observing growth
features effectively requires brightfield illumination, immersion to reduce surface
reflections, and polarised light to reveal twinning and strain.

Colour zoning results from variations in trace element concentration during growth
^[3]:

Formation:

• Changes in fluid chemistry during crystallization
• Different growth faces incorporate elements at different rates
• Temperature or pressure fluctuations

Types:

• Straight zoning - Parallel bands following crystal faces
• Angular zoning - Follows hexagonal or other crystal geometry
• Irregular zoning - Patchy, non-geometric distribution
• Sector zoning - Different colours in different growth sectors

Hollow channels formed parallel to the crystal's c-axis during growth:

Characteristics:

• Elongated, tube-like cavities
• Usually parallel to main growth direction
• May contain fluid or be empty
• Can create "rain-like" appearance

Common in:

• Tourmaline (very characteristic)
• Aquamarine (parallel tubes)
• Some garnets (rare)

Twinning occurs when crystal lattices join in specific orientations.
Polysynthetic twinning in corundum leaves parallel twin planes ^[3].

Types:

• Contact twins - Two crystals joined at a plane
• Penetration twins - Crystals interpenetrate
• Polysynthetic twins - Multiple parallel twin planes

Appearance under magnification:

• Reflection planes visible internally
• Interference colours under polarized light
• Step-like features at twin boundaries

Phantom inclusions show earlier growth stages preserved inside the crystal:

Formation:

• Crystal growth temporarily stops
• Surface accumulates dust or mineral coating
• Growth resumes, encasing earlier form

Appearance:

• Ghost-like outline of earlier crystal
• Often shows different colour or clarity
• Demonstrates crystal's growth history

Common in:

• Quartz (chlorite or iron oxide phantoms)
• Fluorite (colour change phantoms)
• Occasionally in other minerals

Internal graining results from structural irregularities during growth:

Types:

• Whitish graining - Light-coloured lines or planes
• Reflective graining - Shiny internal reflections
• Coloured graining - Associated with colour distribution

Most significant in diamonds:

• Affects clarity grading
• Can indicate plastic deformation
• May show strain patterns

Visible under crossed polarizers, strain patterns reveal internal stress:

Causes:

• Rapid temperature changes during formation
• Pressure from included crystals
• Tectonic stress during geological history

Observation:

• Best seen with polarized light
• Appears as interference colours
• Patterns radiate from stress sources

Chevron or zigzag zoning in hydrothermal synthetic emeralds is diagnostic of synthetic
origin ^[1].