Conchoidal fracture is a mode of breakage in which a brittle material splits along smooth, curved surfaces rather than along crystallographic cleavage planes, producing a series of concentric ridges and hollows that closely resemble the interior of a clamshell. The term derives from the Greek konkhē, meaning shell. It is among the most diagnostically useful physical properties in gemmology and mineralogy, appearing prominently in quartz, obsidian, diamond, and glass, as well as in a wide range of other gemstones that lack well-developed cleavage in a given direction.

Mechanism and Appearance

When stress is applied to a homogeneous, isotropic, or near-isotropic material — one whose internal bonding is roughly equal in all directions — the crack front propagates outward from the point of impact in a roughly hemispherical wave. The resulting surface displays a central point of origin (the hackle or percussion point), surrounded by smooth, undulating ripples that diminish in amplitude with distance. Under magnification, these ripples are unmistakable: they curve gently, maintain consistent spacing near the origin, and may show fine radial striations called Wallner lines where secondary stress waves have intersected the primary crack front. The overall surface has a glassy, almost luminous quality that distinguishes it sharply from the flat, mirror-like faces of true cleavage.

Which Gemstones Show It

Conchoidal fracture is most classically associated with:

• Quartz — all varieties, from rock crystal to amethyst to chalcedony, fracture conchoidally because the trigonal crystal structure offers no dominant cleavage. Prehistoric toolmakers exploited this property to knap flint (a cryptocrystalline quartz) into blades with predictable, controllable edges.
• Obsidian — volcanic glass is structurally amorphous and therefore entirely without cleavage; its fracture is almost perfectly conchoidal, producing the sharpest natural edges known.
• Diamond — although diamond possesses perfect octahedral cleavage in four directions, fracture surfaces that do not coincide with those cleavage planes are conchoidal. Cutters and polishers must distinguish between a cleavage and a conchoidal fracture when assessing damage.
• Opal — being amorphous hydrated silica, opal fractures conchoidally in all directions, with no cleavage whatsoever.
• Garnets and spinel — these species have indistinct or absent cleavage and typically fracture conchoidally to subconchoidal.

Conchoidal Fracture as an Inclusion Feature

Within a polished gemstone, healed or partially healed conchoidal fractures appear as curved, two-dimensional breaks — commonly called feathers — visible under magnification. A feather is essentially a fracture plane that has been partially re-sealed by secondary mineralisation or by the pressure and heat of geological time, leaving a thin film of fluid or gas along the crack surface. The curved outline of the feather, and the faint ripple texture sometimes preserved within it, are direct evidence of the conchoidal mechanism. Gemmologists use the shape and orientation of such features to distinguish fractures from cleavages: cleavage breaks are flat and follow crystallographic directions, whereas conchoidal fractures curve freely through the stone regardless of crystal orientation.

Diagnostic Value in Identification

Observing the fracture surface of an unknown specimen — either on a natural break or on a chip — provides immediate information. A smooth, glassy, shell-like surface points toward an amorphous material (glass, opal) or a crystalline species lacking cleavage in that direction. A flat, mirror-like surface with a slight iridescent sheen (known as schiller on cleavage faces) points toward a well-cleaved mineral such as feldspar, topaz, or fluorite. This single observation can rapidly narrow identification possibilities before refractive index or specific gravity testing is performed. In the field, where instruments are unavailable, the character of a fresh break remains one of the most reliable rapid-assessment tools available to a gemmologist.

Practical Implications in the Trade

For the jeweller and the gem dealer, understanding conchoidal fracture has direct commercial consequences. A conchoidal fracture reaching the surface of a polished stone — whether from a blow during setting, a knocking incident in wear, or a pre-existing internal feather that has propagated — cannot be re-polished away without significant loss of weight, because the curved surface dips below the facet plane. In grading reports issued by laboratories such as the GIA, surface-reaching fractures are noted as clarity characteristics and may be described as fractures or feathers depending on their depth and visibility. A large feather that reaches the surface and shows white, reflective curvature under oblique illumination is considered a significant durability risk in stones such as emerald or aquamarine, where conchoidal fractures are common and the material is frequently worn in exposed settings.