Fracture is any break in a gem that does not follow a cleavage plane. Where cleavage leaves flat, smooth surfaces in set directions, fracture leaves curved, rough or irregular surfaces in any direction. The shape of a fracture can help identify a mineral, and fractures inside a stone affect its clarity, durability and value.
Fracture and cleavage compared
Every gem can fracture. Only some can cleave. Cleavage depends on planes of weak bonding in the crystal structure, so it repeats in the same directions in every specimen of a mineral. Fracture happens when a stone breaks across those planes, or when it has no weak planes at all.
| Feature | Cleavage | Fracture |
|---|---|---|
| Surface | Flat, smooth, often mirror-like | Curved, rough or irregular |
| Direction | Fixed by crystal structure | Any direction |
| Repeats | Yes, parallel steps along the same plane | No set pattern |
| Typical gems | Diamond, topaz, kunzite, feldspar | Quartz, glass, garnet, opal |
A stone can show both. Diamond has cleavage in four directions but fractures in other directions. The cleavage page covers how those flat breaks form.
Types of fracture
Conchoidal
A conchoidal fracture leaves a smooth, curved, shell-like surface with concentric ripple marks. The name comes from the Greek for mussel shell. It is the typical break of materials with no cleavage and an even structure: glass, obsidian, quartz and many single-crystal gems. Prehistoric toolmakers relied on conchoidal fracture to flake flint and obsidian into sharp blades.
On a cut gem, a small conchoidal chip on the girdle or a facet junction is a common sign of a knock. Glass imitations often show these chips along facet edges, and the chip surfaces have a glassy luster. The glass in jewelry page lists other clues that a stone is glass.
Uneven or irregular
An uneven fracture leaves a rough surface with no regular shape. It is common in minerals whose structure is less uniform, and in gems that are full of inclusions. Many garnets and corundum crystals break this way.
Splintery or fibrous
A splintery fracture produces sharp, elongated splinters or fibers. It shows up in minerals made of fibrous crystals, such as nephrite jade, whose interwoven fibers give it great toughness.
Grainy
A grainy fracture leaves a sugary surface. It occurs in aggregates made of many small grains, such as some chalcedony, turquoise and quartzite. The break runs around the grains instead of through them.
Hackly
A hackly fracture leaves jagged, sharp edges. It is the typical break of native metals such as gold, silver and copper, and it is rare in gems.
Describing fracture surfaces
Fracture surfaces have their own luster. Gemologists sometimes describe it alongside the fracture type: a vitreous conchoidal fracture on quartz, a waxy one on chalcedony, a resinous one on amber. This helps on rough or damaged material, where the polished surface no longer tells the full story. The gemstone optics page defines the main luster terms.
Fractures as inclusions
Inside a gem, a fracture is a clarity feature. In diamond grading, a small internal crack is called a feather. Larger fractures that reach the surface can affect durability, especially near a girdle or a prong. The diamond inclusion library describes feathers and related features.
In colored stones, fractures are judged by their size, position and visibility. A small healed fracture deep in a sapphire may hardly matter. An open fracture running to the surface of an emerald is both a clarity issue and a weak point.
Some fractures heal naturally while the crystal is still growing. Fluids in the crack deposit new crystal material, leaving a pattern of tiny droplets often called a fingerprint. These healed fractures are common in corundum, quartz and topaz. Heat treatment can also heal fractures, sometimes with the help of fluxes, a topic covered on the ruby treatments page.
Fracture-filled gems
Filling fractures with a transparent substance makes them less visible. Light reflects strongly from the air inside an open crack. Replacing the air with a material that has a refractive index close to the gem's cuts down that reflection, so the crack seems to fade.
Common fracture fillings include:
- Oils and resins in emerald. Cedarwood oil has been used for centuries, and synthetic resins are also common. Almost all emeralds on the market have been treated this way to some degree.
- Lead glass in ruby. Low-grade, heavily fractured corundum is filled with a high-lead glass. The result is sometimes called composite ruby, since a large part of the stone can be glass.
- Glass or resin in diamond. Fracture-filled diamonds can show a flash of color, often orange, blue or pink, when tilted. The diamond treatments page describes this flash effect.
Fillings change how a gem should be cared for. Heat, solvents, ultrasonic cleaners and steam can damage or remove them. Lead-glass filled rubies can be etched by common acids, including some found in lemon juice and household cleaners. Fillings must be disclosed, and the gemstone treatments page explains disclosure norms.
Fracture and durability
A stone's resistance to breaking is its toughness. Tough materials such as jade resist fracture, while brittle gems such as zircon and tanzanite chip more easily. Existing fractures make any stone more fragile, because they give a crack a head start. The gemstone durability guide covers which gems suit rings worn every day.
Frequently asked questions
What is a conchoidal fracture?
It is a smooth, curved break with concentric ridges, like the inside of a shell. Glass, quartz and obsidian commonly break this way.
Is a fracture the same as a crack?
In gemology, yes. A fracture is any break that does not follow cleavage. In diamonds, small internal fractures are called feathers.
Are fracture-filled gems less durable?
The filler does not make the stone stronger, and heat or chemicals can damage it. Filled gems need gentler cleaning and repair.
How can I tell if a gem has a filled fracture?
A gemologist looks for flash effects, trapped gas bubbles and flow lines in the filler under magnification. Lab reports state when a filling has been detected.