Fracture in Gemstones: Conchoidal, Even, Uneven & Splintery
Fracturing in Gemstones
The term "fracture" (aka "chipping") describes any irregular breakage that does not occur along smooth cleavage planes that are inherent to the material's crystalline structure. All minerals, including those with "perfect" cleavage exhibit the ability to fracture or shatter when stressed, but when strong cleavage is present the exact nature of the fracture's "joint" can be difficult to ascertain.
This type of fracture, also known as "loss of cohesion," is defined as any separation or mechanical rupture of a material into two or more pieces, under the exertion of stress which is applied to a zone of structural discontinuity. Typically, a fracture has none of the geometric sharpness of cleavage.
Conchoidal fracture in obsidian (photo: R.Weller/Cochise) |
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Splintery fracture in asbestos (photo: public domain) |
Some gemstones can fracture without the entire gem-body separating into two or more pieces. The inclusions these internal fractures create are referred to as feathers, which can form as stress fractures around included crystals that were undigested during crystal growth. Any internal fracture tends to reduce the strength most gemstones, and they can also inhibit the transmission of light causing cloudiness.
Conchoidal, Even, Uneven, & Splintery Fracture
Minerals tend to have a very distinctive fractures which can be easily identified. There are several categories of fracture types, which are descriptive of the way in which the material separates under stress. They are:
- Conchoidal Fracture: A smooth, shell-like, curved fracture that is conically shaped
- Earthy Fracture: Occurs in soft, loosely bound minerals
- Even Fracture: Leaves a smooth, flat surface
- Jagged Fracture (Hackly Fracture): Occurs in native metals such as copper and silver
- Splintery Fracture: Occurs in fibrous and non-fibrous minerals such as kyanite
- Uneven Fracture: Occurs in a wide range of minerals. Leaves a rough, irregular surface
- Subconchoidal Fracture: Similar to conchoidal, but not as curved
The term "conchoidal fracture" (aka "shelly fracture") describes the way in which a brittle mineral or material will break when the break does not follow a natural plane of separation. A conchoidal fracture forms a smooth, shell-like, curved fracture joint that has conically shaped (Hertzian cone) ripples across the break. Conchoidal fractures typically occurs in fine-grained minerals that have no cleavage such as quartz, and is common in glass, flint or obsidian.
When the break occurs a swelling appears at the point of impact, which is called the "bulb of percussion." From this point, a shock-wave pattern expands outward from the proximal end of the break towards the distal end, forming concentric ripples across the surface. The nature of the conchoidal fracture in materials like flint and obsidian made it easy for primitive man to form arrowheads and cutting tools (flint knapping). A "subconchoidal" fracture would be something in-between a conchoidal fracture and an "even" fracture. The convex side of the fracture is referred to as the "dorsal" and the concave side is referred to as the "medial."
Plumose Structure
When a mineral fractures due to brittle deformation, it can form a feather-like pattern on the surface of the joint which is known as a "plumose structure." A plumose structure is a record of a fracture's propagation direction (pume axis), and is seen primarily in sedimentary rock. The structure itself begins at the bulb of percussion and radiates outward past the "arrest line," with each concentric plume terminating in a "twist hackle" and "hackle fringe."
Ductile Fracture (Metals)
A "ductile fracture" occurs when extensive plastic deformation takes place prior to the mechanical rupture. Noble metals with a high purity can withstand large amounts deformation (plasticity) before fracture will occur.
The strain at which the fracture happens is typically determined by the purity of the material. In a ductile fracture, some of the energy from stress concentrations at the crack tips is dissipated by plastic deformation before the crack actually propagates into a total failure.
Bibliography on Gemstone Fracture
1. Edward Jay Epstein The Diamond Cut  . www.edwardjayepstein.com
2. Philip A. Candela Fractures and Fracturing . www.geol.umd.edu
3. M. Leeder, M. Arlucea Physical Processes in Earth and Environmental Sciences . Wiley-Blackwell
4. Zangerl, Loew, Eberhardt, Brittle discontinuities in anisotropic crystals . Birkhäuser Basel
5. Callan Bentley, Plumose Structure . www.nvcc.edu
6. Bill Long, Fracture and Cleavage . www.drbilllong.com
7. Terra Australis, Stone Fracture, Knapping . www.epress.anu.edu.au
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