Minerals with perfect cleavage will cleave leaving no rough surface; a full, smooth plane is formed where the crystal breaks. Minerals with good cleavage also leave smooth surfaces, but often leave over minor residual rough surfaces. On minerals with poor cleavage, the smooth crystal edge is not very visible, since the rough surface is dominant. If a mineral exhibits cleavage, but it so poor that it is hardly noticeable, it has “indiscernible” cleavage. Minerals with no cleavage never exhibit any cleavage, thus broken surfaces are fractured and rough. Quality of cleavage can be categorized into five qualities: Perfect, Good, Poor, Indiscernible (Indistinct) and None. Categorization of cleavage qualities is not scientifically affirmed. The above categorization is used by most mineral references, but some guides categorize cleavage in three or four groups, and may give them different names, such as “excellent” and “distinct”.
Many minerals exhibit cleavage only on one side, and some may exhibit different quality cleavage on different crystal sides. The following criteria may be expected when analyzing the cleavage of any particular mineral: One Direction, Two Directions, Three Directions and All Directions. These identify how many “directions”, or planes, the crystal is exhibiting the cleavage on. Each direction signifies the two opposite sides of a three-dimensional figure, (since opposite sides will always exhibit the same cleavage properties). If a mineral has cleavage in three directions, then every side of the mineral has cleavage (i.e. length, width, and height). If a mineral occurs in modified crystals with more than six sides (i.e. an octahedron) and exhibits cleavage on all the sides, than it has cleavage in “all directions”.
Some gemstones, such as the many varieties of quartz, have no cleavage at all. The same is true for opal, jasper, agate, jade and turquoise. Among the finer gems, members of the corundum family (sapphire and ruby) are notable for having no cleavage. The list of gemstones with perfect cleavage along one or more planes is a long one. It includes diamond, topaz, fluorite, spodumene (including hiddenite and kunzite), kyanite, and all the feldspar varieties (including moonstone, orthoclase, amazonite and andesine), tanzanite and other members of the zoisite family.
Combining the cleavage level together with the number of sides will measure the cleavage of a mineral. For example, if a mineral has Good Cleavage, Two Directions, this means that it has good cleavage on four out of six sides (while the other two sides exhibit no cleavage). If a mineral has Perfect Cleavage, One Direction; Poor Cleavage, Two Directions, this means that the mineral has perfect cleavage on two sides, and poor cleavage on the other four.
If you own a gemstone with perfect cleavage, such as diamond, topaz or tanzanite, you needn’t be unduly worried about splitting your gemstone. The risk of cleavage is much higher during the cutting and polishing process, and when the gem is set. Gems with perfect cleavage are usually mounted in protective settings or set in earrings or pendants that are less likely to be subjected to a sharp blow that could result in cleavage. Diamond, with perfect cleavage on four planes, is worn in millions of rings on a daily basis without problem.