Indistinct cleavage is the lowest tier in the standard mineralogical and gemmological classification of cleavage, sitting below "perfect," "good," "distinct" and "poor" in the descending series. The term describes a crystallographic plane along which a mineral will preferentially break, but only when subjected to substantial force and only with a fracture surface that is not flat or smooth in the way produced by a higher-grade cleavage. In practice, an indistinct cleavage is detectable on careful examination of broken fragments but does not represent a material durability concern in cut-stone form.

The cleavage classification

Cleavage refers to the tendency of a crystalline mineral to split along specific planes determined by the weakest bonds in its atomic structure. The standard descriptive series, as used in mineralogical and gemmological texts (Hurlbut and Klein, Manual of Mineralogy; Webster, Gems; GIA reference materials), runs from perfect through to absent:

• Perfect cleavage produces flat, mirror-like cleavage surfaces with little force, as in the basal cleavage of mica or the four-direction octahedral cleavage of diamond.
• Good cleavage produces flat surfaces under modest force but with some imperfection, as in the prismatic cleavage of topaz or the basal cleavage of feldspar.
• Distinct cleavage is detectable as preferential planes of breakage but with surfaces that are noticeably less smooth than "good," as in many of the pyroxenes and amphiboles.
• Indistinct cleavage indicates planes along which the mineral has some preferential weakness but where breakage produces irregular surfaces and the cleavage is detectable only on close inspection of broken fragments.
• Poor cleavage is similar to indistinct, with the planes barely identifiable.
• None / absent indicates that the mineral fractures conchoidally or irregularly with no preferential plane of breakage.

Gem species with indistinct cleavage

A number of important gem species are classified as having indistinct cleavage. These include corundum (sapphire and ruby), which has no true cleavage but exhibits parting along certain twin and growth planes that is sometimes described in the literature as an indistinct or weak parting; spinel, which has indistinct octahedral parting; quartz, which has very poor or indistinct rhombohedral cleavage; and a number of other species in which the bond structure produces some preferential planes but no plane strong enough to dominate the fracture behaviour.

The practical consequence is that these species can be cut, set and worn with no special precaution against cleavage. The fracture behaviour, when impact occurs, is generally conchoidal or irregular rather than along a flat plane, and the resulting damage is typically a chip rather than a clean split.

Compare with significant cleavage

The contrast is sharp with species that have perfect or good cleavage. Diamond's perfect octahedral cleavage in four directions is the classical case: a cutter intentionally exploits the cleavage planes during preforming, and a sharp impact along a cleavage plane can split a finished diamond cleanly. Topaz has perfect basal cleavage that is a routine concern in the workshop and that defines how the species is set and worn; rings using topaz are vulnerable to impact damage along the cleavage. Feldspar (orthoclase, microcline, plagioclase, including the gem varieties moonstone, sunstone and labradorite) has two good cleavages at near-90 degrees that are the principal durability concern in the species. Fluorite has perfect octahedral cleavage that limits its use in jewellery to protected settings.

Indistinct cleavage species can be treated, for setting and wear purposes, as effectively cleavage-free. The bench jeweller does not need to plan the setting orientation around the cleavage planes, and the customer can wear the stone in standard settings without incurring the risk profile that perfect-cleavage species require.

Reporting and laboratory practice

Cleavage information is reported on most gemmological identification reports as part of the standard mineral description and is one of the diagnostic data points used in identification when combined with refractive index, specific gravity, optical character and other tests. The reporting language is generally drawn from the standard cleavage tier scale, though some laboratories use additional qualifiers such as "weak," "strong" or numerical-direction descriptors (e.g., one direction perfect, two directions good).

For the buyer or jewellery manufacturer, the relevant operational point is to consult the cleavage character of any unfamiliar species before specifying setting style and intended wear. For corundum, spinel, and quartz at indistinct cleavage, no special accommodation is required. For diamond, topaz, feldspar and other species with significant cleavage, setting orientation, mount design and wear category should be planned with the cleavage planes in mind.