Cleavage.

Cleavage is how a mineral chooses to break. Inside every crystal the atoms are bonded more tightly in some directions than in others, and when a stone is struck the break tends to follow the planes where the bonds are weakest. A mineral with perfect cleavage will split into smooth flat sheets or blocks. One with poor or no cleavage breaks unevenly, in what is called a conchoidal fracture (the curved shell-like surface seen in obsidian or flint). Mineralogists describe cleavage by two measures: quality (perfect, good, distinct, indistinct, poor, none) and direction (one, two, three, four, or six planes), often noting the angles between those planes as well.

Examples across the mineral kingdom

Mica has perfect cleavage in one direction, which is why it peels into thin transparent sheets that were once cut into oven and lantern windows. Halite (rock salt) and galena cleave perfectly in three directions at right angles, producing the small cubes seen in school collections. Calcite cleaves in three directions but at oblique angles, producing rhombohedral fragments rather than cubes; this is why a broken piece of Iceland spar can be lined up flat against a table on three different faces. Fluorite has perfect cleavage in four directions and breaks into octahedra so neatly that the offcuts are sold as tumbled "octahedrons." Diamond, despite sitting at 10 on the Mohs scale, has perfect cleavage in four directions parallel to its octahedral faces, which is the property a diamond cutter uses to split a rough stone in two with a single tap.

Feldspars, including moonstone, labradorite, sunstone, and amazonite, have good cleavage in two directions that meet at close to ninety degrees (the name orthoclase means "straight break"). A hard knock on a corner can produce a sudden chip that runs flat along one of those internal planes. Topaz has perfect cleavage in one direction, parallel to its base, which is why faceted topaz is set in protective mounts. Quartz, by contrast, has no cleavage at all and breaks with a conchoidal fracture, one of the reasons quartz pebbles tumble so beautifully in rivers.

Why it matters in practice

A common misconception is that hardness and toughness are the same thing. They are not. Hardness measures resistance to scratching (the Mohs scale). Toughness measures resistance to breaking, and cleavage is a major part of that story. Diamond is the hardest natural mineral but is not especially tough; a sharp blow on a cleavage plane can split it cleanly. Jade, which sits lower on the Mohs scale at around 6 to 7, is famously tough because its interlocking fibrous structure has no good cleavage to break along.

For anyone setting stones in jewellery or simply storing a collection, cleavage is the property worth respecting. Stones with good cleavage want padded storage and gentle handling. Kyanite, fluorite, topaz, calcite, and the feldspars all benefit from being kept apart from each other and from harder companions in the same drawer. A bezel setting protects a cleavage-prone stone better than a four-claw setting, and ultrasonic cleaning is best avoided for anything with perfect cleavage, since vibration can encourage an existing internal plane to open. Hardness alone does not tell the whole story. A 7 on the Mohs scale with no cleavage will outlast a 9 with perfect cleavage in everyday wear, which is one reason quartz endures so well as a daily stone.