Cleavage cracks
These occur in the direction of the cleavage planes parallel to the four octahedral faces.
Cleavage Crack - A break parallel to a cleavage plane. It is characterized by a two-dimensional nature; intersections with facets are usually straight lines.
Some minerals may exhibit a pearly luster on cleavage cracks parallel and below the reflecting surface of a mineral.
Inclusions, fractures, and incipient cleavage cracks all fall under the general heading of "flaws" that affect a diamonds clarity. In grading, no distinction is made among them, except with respect to the extent each affects appearance.
Incipient cleavage cracks often are visible inside the gemstone. Rainbow moonstone is a close relative of labradorite.
Usually the cleavage crack extends to the surface. It is the most damaging kind of imperfection in a diamond, since it affects durability as well as beauty.
elongated primary growth tubes that often house liquids containing gas bubbles and microlite crystals; short negative crystals; "snow-stars" formed by microlite crystals surrounded by droplets of liquid on cleavage planes; cleavage cracks; ilminite; ...
 See also: Cleavage, Color, Gemstone, Crystal, Hardness
  
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