Cleavage

Gemstones do break. Some come apart with relative ease, while others require the shock of an energetic blow or the unusual stresses that result from sudden and extreme cooling or heating. The ease with which they break up depends upon the strengths of the bonds between atoms holding them together. If the breakage is irregular, leaving uneven or jagged surfaces at the break, it is called "fracture." At times, with different minerals, the breakage will follow definite directions. The crystal structure splits where the bonding is weakest between certain planes of atoms in the internal arrangement. This causes the broken surfaces to be flat faces parallel to planes of atoms which are parallel in turn to possible crystal faces. Topaz, for example, always breaks in a manner showing flat surfaces or cleavage planes that develop in one same direction through the structure. Other minerals may exhibit cleavage in two, three, or even four different directions through the crystal structure. Feldspar cleaves in two directions which are at right angles to each other. Spodumene also has a two-directional cleavage, but the two are not quite at right angles. Since the cleavages of different kinds of gems vary considerably, they can be used to help identify the jewelry stones.