Cleavage, Parting, and Fracture: Understanding How Gemstones Break

A diamond is the hardest natural material on earth — and yet a skilled setter has split one cleanly in two with a single blow. A tanzanite is a spectacular gem of intense violet-blue — and yet it demands extreme care during setting because a glancing knock can crack it. An opal is a gem of incomparable beauty — and yet it can fracture from the stress of a sudden temperature change. How a gemstone breaks is a property as important as how it shines. Cleavage, parting, and fracture are the three ways a gem can fail structurally, and understanding each is essential for anyone who buys, sells, sets, or wears fine jewellery.

This article explains the mechanics behind each type of structural failure, which gemstones are most vulnerable, and what this means for jewellery professionals, setters, and customers who want their pieces to last.

What Is Cleavage?

Cleavage is the tendency of a crystalline mineral to break along flat, parallel planes defined by its atomic structure. These planes are directions of weakness — where the bonds between atoms are relatively weak — and once a stone is struck or stressed along such a plane, it tends to split cleanly and predictably.

Cleavage is described by two characteristics: direction and quality. Direction refers to which crystallographic plane is involved (basal, prismatic, pinacoidal, etc.). Quality is described as perfect, good, fair, or poor, indicating how cleanly the stone splits.

Diamond: Perfect Octahedral Cleavage

Diamond has four directions of perfect cleavage, each parallel to a face of the octahedral crystal form. This cleavage was the foundation of the entire diamond cutting industry before modern saw technology — early cutters split rough diamonds along cleavage planes to remove unwanted sections or create manageable pieces from larger crystals. The process (called “cleaving”) required a skilled artisan who understood the stone’s internal orientation perfectly.

Today, cleavage remains a practical concern for setters and jewellers. A diamond set in a bezel that applies uneven pressure, or struck at precisely the wrong angle, can still cleave cleanly. An I1 diamond with a large feather running parallel to a cleavage direction has a structural vulnerability that a setter must understand.

Topaz: Perfect Basal Cleavage

Topaz has one direction of perfect basal cleavage — perpendicular to the crystal’s length. This means that a blow to the end of a topaz crystal, or pressure applied across the base, can cause it to cleave. In jewellery, this means topaz requires protective settings and cannot be set by pressure-setting techniques (tension settings, bezel settings that apply clamp pressure perpendicular to the cleavage) without risk. This cleavage direction also explains why topaz crystals are often found with a characteristic glassy, flat surface — a natural cleavage face.

Feldspar Family: Including Moonstone

The feldspar family — which includes moonstone, labradorite, and sunstone — has two directions of perfect cleavage intersecting at approximately 90 degrees. This makes feldspars relatively fragile compared to their hardness (6–6.5 on the Mohs scale) might suggest. A moonstone, despite its beautiful adularescence and romantic appeal, requires careful setting and gentle wear. Bezel settings rather than prong settings are generally preferred.

What Is Parting?

Parting is a phenomenon that resembles cleavage but has a different cause. Rather than inherent atomic weakness in one direction, parting occurs along planes of structural irregularity created during crystal growth — most commonly twinning planes (where two crystals grew together in a mirror-image relationship).

The distinction between cleavage and parting matters to gemologists and cutters, but is largely academic from a practical standpoint: both produce flat, planar breaks that can compromise a stone. Corundum (ruby and sapphire) shows parting rather than true cleavage — one reason why the cutting and setting of these stones requires knowledge of their internal structure.

What Is Fracture?

Fracture is any break that does not occur along a flat cleavage or parting plane. Unlike cleavage, which follows atomic structure, fracture produces irregular surfaces that may be conchoidal (shell-like, with curved ridges — characteristic of glass and quartz), hackly (jagged, irregular), splintery, or fibrous.

Gemstones without cleavage break by fracture, and many gemologists consider this a durability advantage: the break, when it occurs, is less catastrophic and less predictable in direction. A quartz crystal struck hard will fracture — but it will not cleave cleanly along an entire plane, making the break more contained.

Conchoidal Fracture

The conchoidal (shell-like) fracture of glass, quartz, and opal is one of the most recognisable in gemology. Under magnification, the curved ridges radiating from the impact point look remarkably like the growth rings on a bivalve shell. Obsidian, a volcanic glass, fractures conchoidally so reliably that early humans used it for cutting tools. In opal, conchoidal fracture combined with natural stress from water loss can cause craze lines — fine fracture networks that develop over time.

Practical Implications for Jewellery Professionals

Setting Considerations

Knowledge of cleavage and parting is essential for anyone who sets gemstones. The key rules:

Never apply perpendicular pressure across a cleavage direction during setting.

Use protective settings (bezels, halos, low-profile prongs) for stones with perfect cleavage.

Assess the direction of significant feathers before setting — a feather parallel to a cleavage direction is a heightened risk.

Topaz, tanzanite, and kyanite require particular care; tourmaline and garnet are generally more forgiving.

Customer Education

Customers who understand their stone’s structural properties make better choices about ring styles, wear habits, and care. A customer buying a tanzanite engagement ring deserves to know that tanzanite has perfect cleavage in one direction and is better suited to a protective setting than a high-prong solitaire. This knowledge is not meant to discourage the purchase — it is meant to ensure the piece is worn and enjoyed safely for decades.

Repair and Resizing

Stones with perfect cleavage can be vulnerable during heat application (soldering) if the setting does not provide even support. Temperature changes create differential expansion stresses; if a stone is gripped unevenly in its setting when heat is applied, the stress can exploit a cleavage plane. Some stones (notably emeralds with their oil-filled fractures and tanzanite with its cleavage) are routinely removed from settings before any repair work is done.

Key Takeaways

Cleavage is breakage along defined atomic planes of weakness — present in diamond (octahedral), topaz (basal), and feldspars (two directions).

Parting resembles cleavage but occurs along twinning planes; corundum shows parting, not true cleavage.

Fracture is any irregular break not along a cleavage plane — conchoidal fracture is characteristic of quartz and glass.

Setters must know cleavage directions to avoid applying pressure that exploits structural weakness.

Customers deserve to know their stone’s durability properties — this is service, not salesmanship.

Stones with perfect cleavage (topaz, tanzanite) benefit from protective settings and careful everyday wear habits.