Feather
Feather
A fracture inclusion characterised by its white, plume-like appearance under magnification
A feather is a fracture or partial break within a gemstone that appears white and translucent under magnification, its delicate, branching reflectance resembling the vane of a bird's feather. The term is used across both diamond and coloured-gemstone grading and is among the most frequently encountered clarity characteristics in natural stones. Feathers may be entirely internal or surface-reaching, healed or open, and their significance to a stone's value and durability varies considerably depending on size, orientation, depth, and the species in question.
Optical Cause
The characteristic white appearance arises from light interference along the plane of the break. Where two surfaces of a fracture are in near-contact, incident light is partially reflected and partially scattered at the interface, producing the milky or iridescent sheen that distinguishes a feather from a transparent cleavage crack or a dark mineral inclusion. In stones with strong cleavage — octahedral in diamond, basal in topaz, rhombohedral in calcite-bearing assemblages — feathers frequently follow crystallographic planes, giving them a relatively flat, regular geometry. In stones with conchoidal or irregular fracture, such as quartz or glass, feathers tend to be more curved and irregular in outline.
Healed and Open Feathers
Gemmologists distinguish between open feathers, in which the fracture surfaces remain separated and accessible, and healed feathers (sometimes called healed fractures or fingerprints), in which secondary material — typically fluid inclusions arranged in planar arrays — has partially or fully sealed the break. Healed feathers often display iridescent interference colours, particularly in corundum, and may be mistaken for fingerprint inclusions, which are themselves a subset of healed fractures. The distinction matters practically: an open, surface-reaching feather is a pathway for fracture-filling treatments and for the ingress of ultrasonic cleaning fluids, acids, or oils, whereas a fully healed internal feather poses no such risk.
Significance in Diamond Grading
In the GIA clarity-grading system for diamonds, feathers are explicitly named as one of the standard clarity characteristics plotted on grading reports. A small feather confined well within the stone may be consistent with grades as high as VS1 or VS2; a feather that is large, surface-reaching, or oriented perpendicular to the table may reduce a stone to SI or I clarity. Of particular concern are feathers that extend to a girdle edge or a facet junction, where the mechanical stress of setting or wear can propagate the break. GIA graders assess feathers not only for size and position but for their potential impact on durability — a consideration noted explicitly in GIA's published clarity-grading standards.
Significance in Coloured Gemstones
In coloured gemstones, the term feather is used descriptively rather than as a formal grade-defining characteristic, since no single universally adopted clarity scale governs the trade in the way the GIA D-to-Z and FL-to-I3 systems govern diamonds. Nevertheless, feathers are among the most commercially important clarity features in ruby, sapphire, and emerald. In emerald, where the jardin — the characteristic internal landscape of fractures, fluid inclusions, and mineral crystals — is accepted as part of the stone's identity, feathers are evaluated in the context of the overall clarity picture rather than in isolation. Laboratories such as Gübelin and SSEF note the presence and extent of fractures when assessing whether fracture-filling treatment (typically with resin or oil) has been applied, since filled feathers are a primary indicator of such treatment.
In ruby and sapphire, surface-reaching feathers are of concern both for durability and because they may have been treated with lead-glass filling, a practice that dramatically improves apparent clarity but substantially reduces value. Major laboratories — GIA, Lotus Gemology, Gübelin — routinely comment on the presence of filled fractures in their reports, and the extent of fracture filling is a key determinant of a coloured stone's disclosure category.
Durability Considerations
The practical risk posed by a feather depends on several factors: its size relative to the stone, whether it reaches a surface, its orientation relative to the direction of likely mechanical stress, and the hardness and toughness of the species. A small internal feather in a sapphire (Mohs hardness 9, no cleavage) is of minimal concern; the same feather in a topaz (Mohs hardness 8, perfect basal cleavage) or an emerald (Mohs hardness 7.5–8, poor toughness) warrants greater caution. Stones with prominent surface-reaching feathers are generally not recommended for rings or bracelets subject to daily impact, and setters should be advised to avoid placing prongs or bezel edges directly over such features.
In the Trade
The word "feather" is preferred in professional gemmological and grading contexts over the more alarming lay terms "crack" or "fracture," though the underlying phenomenon is the same. Auction catalogues and laboratory reports use the term neutrally and descriptively. Buyers should be aware that the presence of feathers does not automatically disqualify a stone from high value — many fine rubies and emeralds of museum quality contain prominent fractures — but that the nature, extent, and treatment history of those feathers are material facts requiring disclosure.