Feathers & Fingerprints

Feathers and fingerprints are healed or partially healed fractures within gemstones,
formed when cracks developed during or after crystal growth were subsequently
infiltrated and partially sealed by circulating mineralising fluids. A feather is
an irregular fracture plane retaining a wispy, veil-like appearance, sometimes showing
iridescent thin-film interference from residual fluid at the healed surface. A
fingerprint inclusion is a more completely healed fracture in which networks of
residual fluid droplets form maze-like patterns resembling the loops and whorls of
a human fingerprint, systematically documented for spinel and corundum by Gübelin
and Koivula. Their gemmological importance is twofold: natural fingerprints confirm
growth in a fluid-bearing environment (ruling out many synthetic methods), while
flux fingerprints (fuzzier, with glassy residue) indicate high-temperature
flux-assisted heat treatment. In emerald, the dense network of surface-reaching
fractures known as the "jardin" (French: garden) is species-characteristic, making
virtually all commercial emeralds candidates for fracture filling with cedar oil
or resin, a treatment requiring disclosure at every point of sale.

Feathers are internal fractures that have a wispy, feathery appearance ^[1].
They may show iridescence from thin-film interference:

Characteristics:

• Appear as wispy, irregular planes
• May show iridescence from thin-film interference
• Can affect durability if reaching surface
• Common term used in diamond grading

Formation:

• Stress during crystal growth
• Post-formation stress or shock
• Partial healing may occur

Fingerprint inclusions are healed fractures with characteristic patterns
^[2]. They form when a crack is subsequently
healed by mineralising fluids:

Appearance:

• Networks of tiny fluid remnants
• Pattern resembles human fingerprints
• Often show maze-like or net-like design
• May contain scattered droplets

Formation process:

1. Fracture develops in crystal
2. Fluids enter the fracture
3. Crystal partially heals around fluid
4. Trapped fluid forms fingerprint pattern

Liquid feathers are partially healed fractures still containing significant fluid:

Characteristics:

• Feather-like fracture outline
• Visible fluid within the fracture
• May contain gas bubbles
• Can show movement when tilted

Common in:

• Spessartine garnet (very characteristic)
• Some tourmalines
• Various other gems

Emeralds typically contain a high density of surface-reaching fractures collectively
called the "jardin" (French: garden) ^[3]. These fractures
are largely protogenetic and syngenetic, and their abundance is a species-characteristic
feature. Fracture filling (oiling, resin) is consequently common trade practice.

Stress fractures form around included crystals due to differential expansion:

Formation:

• Included crystal has different thermal expansion
• Temperature changes create stress
• Fractures radiate from inclusion

Examples:

• Zircon halos in corundum and other gems
• Tension cracks around garnet inclusions
• Discoid fractures (lily pads) around chromite in peridot

Lily pad inclusions are diagnostic for peridot:

Description:

• Flat, disc-shaped stress fractures
• Radiate from a central crystal inclusion
• Resemble lily pads on a pond
• Usually surrounding chromite crystals

Diagnostic value:

• Virtually diagnostic for peridot
• Confirms natural origin
• One of most distinctive inclusion types

To observe feathers and fingerprints effectively:

Lighting:

• Darkfield - Best for seeing feathers as bright features
• Fibre-optic - Pinpoint illumination for detail
• Reflected light - Shows surface-reaching fractures

Method:

1. Start with low magnification for overview
2. Increase magnification for pattern detail
3. Rotate stone to see full extent of feature
4. Note whether feature reaches surface