In gemmology, good cleavage describes a crystal's tendency to fracture preferentially along well-defined crystallographic planes with moderate ease, yielding surfaces that are relatively smooth but not as flat or mirror-like as those produced by perfect or excellent cleavage. It occupies the middle ground in the standard cleavage hierarchy — perfect, excellent, good, distinct, indistinct, none — and is a direct expression of the internal atomic architecture of a mineral: wherever bonds between atomic planes are measurably weaker than those within the planes, a cleavage direction exists, and the degree of that weakness determines its quality rating. Good cleavage is neither a flaw nor a virtue in isolation; it is a physical reality that every cutter, setter, and collector must understand and accommodate.

The Cleavage Hierarchy in Context

Mineralogists and gemmologists rank cleavage quality on a descriptive scale that reflects how readily a mineral parts along a given plane and how smooth the resulting surface is. At the top, perfect cleavage — as seen in topaz in one direction, or calcite in three — produces flat, lustrous surfaces with minimal applied force, and the planes are so consistent that they can be mistaken for crystal faces. Excellent cleavage is nearly as reliable. Good cleavage requires somewhat more deliberate force and produces surfaces that, while recognisably planar, may show minor irregularities or steps. Below it, distinct cleavage is visible under magnification but rarely governs fracture behaviour in practice, while indistinct or absent cleavage leaves conchoidal or irregular fracture as the dominant breakage mode.

The distinction between these grades is not merely academic. A gemstone with perfect cleavage in a given direction will part along that plane under the stress of a sharp blow, a thermal shock, or even the pressure of a tight setting; one with good cleavage in the same direction is more forgiving but still vulnerable under concentrated stress. Understanding where a stone falls on this scale informs every stage of its life from rough to finished jewellery.

Crystallographic Basis

Cleavage arises because crystal structures are anisotropic — their properties differ with direction. In any ionic or covalently bonded mineral, certain planes contain a higher density of weaker bonds or a lower density of bonds overall. When mechanical energy is applied, it propagates most efficiently along these planes of least resistance. The quality of the resulting cleavage depends on how pronounced the contrast is between bond strength across the cleavage plane and bond strength within it. Good cleavage indicates a meaningful but not extreme contrast: the planes are well-defined and reproducible, but the energy differential is moderate enough that the stone does not part spontaneously or under trivial stress.

Cleavage is always parallel to a possible crystal face — that is, it corresponds to a lattice plane of low Miller indices — and is described both by its quality (perfect through none) and by the number of directions in which it occurs. A mineral may have good cleavage in one direction only, or in two, three, or more non-equivalent directions simultaneously, each potentially of different quality.

Feldspar: The Defining Example

The feldspar group provides the most frequently cited example of good cleavage in gemmological literature. Feldspars — including the gem varieties labradorite, moonstone, sunstone, orthoclase, and amazonite — are framework silicates built on a three-dimensional network of silicon- and aluminium-oxygen tetrahedra. This structure produces two distinct cleavage directions: one parallel to the basal plane (designated {001}) rated as perfect, and one parallel to the side pinacoid ({010}) rated as good. The two directions intersect at angles close to 90 degrees in orthoclase and at approximately 86 degrees in the plagioclase feldspars, a difference that is itself diagnostic.

In practice, the good cleavage direction in feldspar is reliable enough to influence how cutters orient rough: a skilled lapidary will position the table facet to avoid placing either cleavage direction perpendicular to the girdle, where a blow during setting could split the stone. The characteristic adularescence of moonstone — the billowing, floating light effect — arises from alternating layers of orthoclase and albite on a scale of tens to hundreds of nanometres, and those layers are themselves oriented parallel to the cleavage planes, meaning the optical phenomenon and the mechanical vulnerability share the same crystallographic origin.

Other Gem Species with Good Cleavage

Feldspar is the most prominent example, but good cleavage appears in several other gem materials:

• Enstatite — the magnesium pyroxene occasionally cut as a collector's gem — exhibits good cleavage in two directions parallel to the prism faces, at roughly 88 degrees to one another, a characteristic pyroxene geometry.
• Diopside, another pyroxene, shares a similar two-directional good cleavage and requires care during faceting and setting for the same reasons.
• Rhodonite, a manganese inosilicate prized for its rose-pink colour and black manganese-oxide veining, has good to perfect cleavage in two directions, which complicates cabochon cutting of larger pieces.
• Apatite, occasionally faceted in its blue, green, or violet forms, possesses indistinct to good cleavage parallel to the basal plane, which, combined with its relatively low hardness (Mohs 5), makes it a stone suited to protective settings.

It is worth noting that cleavage quality can vary between directions within the same stone: a mineral might have perfect cleavage in one direction and good cleavage in another. The practical risk is governed by the weakest direction that is most exposed in the finished gem's geometry.

Implications for Cutting and Lapidary Work

Cleavage quality is among the first properties a lapidary assesses when planning the orientation of a rough stone. With good cleavage, the cutter must balance two considerations: exploiting the cleavage planes to remove unwanted material efficiently (a technique called cleaving, distinct from sawing or grinding), and avoiding orientations in the finished gem that leave a cleavage plane vulnerable to the stresses of everyday wear.

For faceted stones, the table facet is ideally positioned so that no cleavage plane runs parallel to it — a parallel cleavage would mean that any blow to the table could propagate a split through the entire stone. Similarly, cleavage planes running perpendicular to the girdle create a risk of splitting during the bezel-setting or prong-tightening process. With good cleavage (as opposed to perfect), these risks are reduced but not eliminated, and experienced setters will apply force gradually and evenly, often warming the metal rather than hammering it cold.

Cabochon cutting of stones with good cleavage is generally less fraught than faceting, because the grinding process is less likely to initiate a cleavage crack than the percussive or directional stress of faceting machinery. Nevertheless, thermal shock during polishing — from abrupt changes in temperature — can propagate along good cleavage planes in susceptible materials.

Implications for Durability and Jewellery Design

Durability in a gemstone is a composite property of hardness, toughness, and stability. Cleavage quality directly affects toughness — the resistance to breakage under impact. A stone with good cleavage in two directions, such as a feldspar, is more susceptible to chipping or splitting than a stone of similar hardness but no cleavage, such as a spinel or a garnet. This does not disqualify feldspars from jewellery use, but it does inform appropriate design choices: bezel settings and protective rub-over mounts are preferable to high-pronged settings that leave the girdle exposed; rings are higher-risk than pendants or earrings; and stones intended for daily wear should be examined periodically for incipient cleavage cracks, which can propagate slowly under repeated minor stress.

The Gemological Institute of America's gem reference materials consistently note that feldspar's good-to-perfect cleavage makes it more suitable for occasional-wear jewellery than for pieces subjected to daily mechanical stress, a position echoed across the professional gemmological literature.

Identification and Laboratory Use

Cleavage quality is a useful diagnostic property in gem identification, particularly when examining rough or broken material. Under magnification, good cleavage surfaces appear as relatively flat, reflective planes that may show faint parallel striations or minor steps. They differ from fracture surfaces, which are curved (conchoidal), uneven, or splintery. In a polished stone, residual cleavage cracks — sometimes called cleavage planes or cleavage traces — may be visible as straight, reflective internal features oriented parallel to the crystallographic planes, distinct from the curved or irregular outlines of fractures or feathers caused by other mechanisms.

When combined with refractive index, specific gravity, and optical character, the number and quality of cleavage directions can help a gemmologist distinguish between species that might otherwise appear similar. The two-directional good-to-perfect cleavage of feldspar, for instance, is an immediate differentiator from quartz, which has no true cleavage and breaks conchoidally.

Further Reading

• GIA Gem Encyclopedia — gia.edu
• IGS: Cleavage, Fracture, and Tenacity — gemsociety.org
• Gems & Gemology Journal — gia.edu