Parting — The Twin-Plane Cousin of Cleavage
Parting — The Twin-Plane Cousin of Cleavage
Planar separation along twin or weakness planes rather than true crystallographic cleavage
Parting is a planar separation within a crystal that resembles cleavage but occurs along twin planes, compositional boundaries, or zones of structural weakness rather than along the true crystallographic cleavage planes that define cleavage proper. In gem materials, parting is most commonly observed in corundum, where rhombohedral or basal parting planes can develop from twinning or from the exsolution of secondary minerals along structural surfaces. Parting is less predictable than cleavage, may only become evident after mechanical stress or heating, and represents an important consideration for cutters and setters working materials prone to it.
Distinction from cleavage
Cleavage is a property of a mineral's crystal structure: certain crystallographic planes have systematically weaker bonding than others, and the crystal will preferentially split along those planes regardless of any specimen-specific history. Cleavage is reproducible across specimens of the same species and is documented in standard mineralogical references — diamond's octahedral cleavage, topaz's basal cleavage, fluorite's octahedral cleavage, and so on.
Parting, by contrast, depends on the specific history of the individual crystal: presence of twinning, segregation of impurities along certain planes, exsolution of minor phases, or stress-induced microfracturing. Two corundum specimens of identical chemistry and crystallography may differ markedly in their parting behaviour because one has a complex twinning history and the other does not. Parting planes, when they develop, can be just as weak as cleavage planes and just as problematic for cutting and setting.
Parting in corundum
Ruby and sapphire are the gem species in which parting is most often a practical concern. Two principal parting directions are recognised: rhombohedral parting, parallel to the rhombohedral faces of the crystal, and basal parting, parallel to the basal pinacoid. Both can develop in twinned crystals or in crystals that have undergone significant heating, including the heat treatment routinely applied to ruby and sapphire to improve colour and clarity. The exsolution of rutile silk along certain crystallographic planes is one source of parting weakness; the redistribution of minor phases during heat treatment can both create and accentuate parting tendencies.
Cutters working corundum check rough carefully for visible twinning planes and parting tendencies before deciding orientation and cutting strategy. A stone with a strong basal parting may need to be oriented so that the parting plane lies away from the table, where stress during setting is less likely to cause separation along the plane.
Practical implications
For setters, parting in corundum (and in other species where it occurs, including pyroxenes such as diopside and some feldspars) means that the stone may be more vulnerable than its hardness alone would suggest. Conservative setting practices — bezel rather than tension setting, careful avoidance of point loading, and avoidance of ultrasonic cleaning for stones known to have strong parting — reduce the risk of stress-induced fracture along parting planes.
In the trade
For Skyjems clients, parting is a behind-the-scenes consideration in our cutting and setting practice rather than a topic that typically arises at the buyer's level. We assess corundum and other parting-prone material for tendency before cutting and again before setting, and we adjust our techniques accordingly. The principal practical advice for owners of fine ruby and sapphire is conservative wear and care, with attention to avoiding sharp impacts along the stone's edges.