Cleavage is the tendency of a crystal to split along flat planes where the bonds between its atoms are weakest. A sharp knock in the right direction can break a gem cleanly along one of these planes, leaving a smooth, mirror-like surface. Diamond, topaz, kunzite and moonstone all cleave, and that matters more for everyday wear than many buyers expect.
Why cleavage happens
A crystal is a repeating three-dimensional pattern of atoms. In some directions, the atoms are held together by strong bonds. In others, the bonds are weaker or fewer cross the plane. When stress is applied, the crystal parts along those weaker planes.
Because cleavage follows the atomic structure, it always runs parallel to a possible crystal face, and every piece of the same mineral cleaves in the same directions. A mineral can have one cleavage direction or several. Mica has one perfect cleavage, which is why it peels into thin sheets. Calcite has three, which is why it breaks into rhombohedral blocks. The crystal systems page explains how structure sets these directions.
Cleavage is different from hardness. A mineral can be very hard and still cleave easily. Diamond is the hardest natural material, and it has perfect cleavage. The Mohs hardness scale page covers the difference between scratch resistance and resistance to breaking.
How cleavage is graded
Mineralogists describe cleavage by how easily it forms and how smooth the resulting surface is:
| Grade | What it means | Gem examples |
|---|---|---|
| Perfect | Splits easily, leaving a smooth, flat surface | Diamond, topaz, kunzite, tanzanite, kyanite, fluorite |
| Distinct or good | Splits fairly readily, surface mostly flat | Titanite, the second cleavage direction in feldspars such as moonstone |
| Indistinct or poor | Rarely seen, surface rough | Beryl (emerald, aquamarine), peridot |
| None | No cleavage planes | Quartz, garnet, spinel, corundum (some crystals show parting) |
Some references use "imperfect" for weaker grades. The terms are descriptive, so different books may grade the same mineral slightly differently.
Gems with no cleavage break by fracture, which produces curved or irregular surfaces. That is covered on the fracture page.
Cleavage in well-known gems
Diamond
Diamond has perfect cleavage in four directions, parallel to the faces of an octahedron. Its carbon atoms are bonded in a three-dimensional network, but fewer bonds cross the octahedral planes than other planes. A hard blow along one of these directions can chip a girdle or split a stone, even though nothing else on Earth can scratch it. Feathers in diamonds often follow cleavage directions, and cleavage cracks are one reason a clarity grader notes where an inclusion sits.
Topaz
Topaz has one perfect basal cleavage, running at right angles to the length of the crystal. Rough topaz crystals often show a flat end where they broke along this plane. A cut topaz knocked against a hard surface can split cleanly, so cutters orient the table away from the cleavage plane, usually by at least a few degrees.
Spodumene, feldspar and others
Kunzite, the pink variety of spodumene, has two perfect cleavages at about 87 and 93 degrees, which makes it notoriously difficult to cut. Moonstone and other feldspars have two cleavages at or near right angles, one perfect and one good. Tanzanite has one perfect cleavage, which is why it should be kept away from ultrasonic cleaners and sudden temperature changes. Fluorite has perfect octahedral cleavage in four directions and is mostly a collector's stone for that reason.
Gems with poor or no cleavage
Corundum (ruby and sapphire) has no true cleavage, though it can show parting, a similar break along twinning planes that only forms in some crystals. Quartz, garnet and spinel have no cleavage at all, which is part of why they hold up well in jewelry.
How cutters use cleavage
For centuries, diamond cutters used cleavage on purpose. A skilled cleaver studied a rough crystal, scored a small groove along a cleavage direction, set a steel blade in the groove and struck it once with a mallet. Done right, the crystal split cleanly in two. Done wrong, it shattered. The Cullinan diamond, the largest gem diamond ever found, was cleaved in 1908 in Amsterdam before cutting. The history of diamond cutting page tells that story.
Diamonds cannot be polished parallel to a cleavage plane, because the surface will not take a polish in that direction. Cutters orient facets a few degrees away from these planes. Sawing and, since the late 20th century, laser cutting have largely replaced cleaving for most diamonds, since they let planners divide rough along any direction. The how diamonds are cut page covers modern methods.
Colored stone cutters also plan around cleavage. A topaz table cut parallel to the basal cleavage would be hard to polish and prone to splitting, and kunzite and kyanite need careful orientation to avoid chipping on the wheel.
Setting and wear risks
Cleavage creates risk at three points in a gem's life:
- During setting, when pressure from prongs or a bezel can start a split.
- During wear, when a knock against a counter or door frame lands in the wrong direction.
- During cleaning or repair, when ultrasonic vibration, steam or the heat of a jeweler's torch stresses an existing feather or cleavage crack.
Protective settings help. A bezel covers the edge of the stone, and a recessed setting keeps it below the top of the ring. For rings worn daily, gems with perfect cleavage are safer in pendants and earrings, or in settings that shield them. The gemstone durability guide and the gemstone settings page give advice for each type of jewelry.
Frequently asked questions
Can a diamond break if it is the hardest mineral?
Yes. Hardness measures scratch resistance, while cleavage is a tendency to split along planes. A sharp blow in a cleavage direction can chip or split a diamond.
What is the difference between cleavage and parting?
Cleavage comes from the crystal's atomic structure and is present in every specimen of that mineral. Parting follows twinning or stress planes and only appears in some specimens, such as some corundum.
Which popular gems have no cleavage?
Quartz, garnet and spinel have no cleavage. Corundum has none either, though some crystals show parting.
Is it safe to clean a gem with cleavage in an ultrasonic cleaner?
It is risky. Vibration can extend an existing cleavage crack, so gems such as topaz, tanzanite and kunzite are better cleaned with warm soapy water and a soft brush.