A fracture is any break in a gemstone or mineral that occurs independently of the material's crystallographic cleavage planes. Unlike cleavage, which produces flat, planar surfaces dictated by the crystal's atomic bonding structure, fractures arise from mechanical stress, thermal shock, or internal strain generated by inclusions, and their surface morphology reflects the physical properties of the host material rather than its symmetry. In gemmology, the term carries two related but distinct meanings: it describes the character of a broken mineral surface as a diagnostic property, and it denotes a specific category of clarity feature encountered during gemstone grading.

Fracture as a Diagnostic Property

When a mineral is struck or cleaved along a non-preferred direction, the resulting surface takes on a characteristic texture that can assist in identification. Gemmologists and mineralogists recognise several principal fracture types:

• Conchoidal — the most diagnostically significant type, producing smooth, curved, shell-like surfaces with concentric ripple marks radiating from the point of impact. Characteristic of glass, obsidian, quartz, and opal, conchoidal fracture arises in materials with no preferred cleavage and an isotropic or near-isotropic bonding structure.
• Uneven — a rough, irregular surface with no consistent curvature, common in corundum (ruby and sapphire) and many garnets.
• Hackly — a jagged, sharp, splintery surface resembling torn metal, most often seen in native metals and some sulphides, rarely encountered in faceted gem materials.
• Splintery — elongated, fibrous splinters produced in materials with a pronounced fibrous structure, such as nephrite jade or some varieties of chrysocolla.

Fracture type is recorded alongside hardness, cleavage, lustre, and specific gravity as a standard physical property in gemmological identification. For materials that lack cleavage entirely — glass, opal, most garnets — fracture morphology becomes particularly useful because it is one of the few surface-texture properties available for examination.

Fracture as a Clarity Feature

Within the context of clarity grading, a fracture is classified as an inclusion — a clarity feature that originates inside the stone, even when it reaches the surface. The Gemological Institute of America's clarity-grading system treats fractures as internal features when they are wholly enclosed, and as blemishes when they break the surface, though in practice the distinction is often one of degree rather than kind, since many fractures extend from an internal point of stress to the girdle or facet surface.

Fractures in faceted gemstones most commonly originate from one of three causes:

• Mechanical stress — impact during cutting, setting, or wear, particularly at thin girdle edges or sharp culet points.
• Thermal shock — rapid temperature change during heat treatment or laser drilling, which can propagate existing micro-fractures or initiate new ones.
• Inclusion-related strain — stress fractures radiating outward from solid inclusions (crystals, needles) whose thermal expansion coefficient differs significantly from that of the host, a phenomenon particularly well documented in ruby and sapphire containing rutile silk or calcite crystals.

Under magnification, fractures in colourless or lightly tinted stones often appear white or silvery due to total internal reflection at the fracture surfaces. In transmitted light they may appear as flat or gently curved planes with an iridescent or liquid-film appearance. When a fracture has been partially healed by secondary mineral deposition — a common occurrence in natural emeralds, where three-phase inclusions and healed fractures filled with fluid or carbonate minerals are characteristic of Colombian material — it may display a distinctive fingerprint-like pattern of minute fluid inclusions arranged along the former fracture plane.

The Feather: A Specific Fracture Type

In trade parlance, the term feather is applied to a fracture — typically a partially healed or reflective internal fracture — that presents a white, wispy, feather-like appearance under reflected light. The feather is among the most commonly encountered clarity features in corundum and is assessed both for its size relative to the stone and for its proximity to the surface, since a feather reaching a vulnerable area of the girdle or a thin facet junction poses a durability risk. Gemmological laboratories routinely note prominent feathers in their reports and may comment on their structural implications.

Fracture Filling as a Treatment

Because fractures that reach the surface are open to intervention, fracture filling has become one of the most commercially significant gemstone treatments. In ruby, glass filling — the introduction of lead-glass or other high-refractive-index substances into surface-reaching fractures — can dramatically improve apparent clarity and transparency. The GIA and other major laboratories identify fracture filling by the presence of gas bubbles, flow structures, and anomalous interference colours within the filled voids, and disclose the degree of filling on their reports. Fracture-filled rubies command substantially lower prices than untreated or heat-only stones of comparable apparent quality. Emeralds are routinely clarity-enhanced with oils, resins, or polymers introduced into surface-reaching fractures, a practice so widespread that it is considered a standard trade treatment; the nature and degree of filling are disclosed on laboratory reports using scales ranging from "none" to "significant."

Durability Implications

The presence, size, and orientation of fractures are primary considerations in assessing a gemstone's durability. A fracture parallel to a facet or running through a thin section of the stone — particularly near the girdle — weakens the stone's resistance to further impact. Stones with prominent fractures require protective settings and careful handling. This is especially pertinent in gems with moderate to good cleavage, such as topaz, where an existing fracture may propagate along a cleavage direction under relatively modest stress.