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An internal fault or imperfection in a gemstone, as distinguished from a blemish, which is a marring of the surface. Flaws may be due to the inclusion of a foreign material, to a small crack or cleavage, or to a liquid-filled cavity.
A flaw usually detracts from the value of a gemstone (especially in a diamond), but less so in a emerald which is almost invariably has some minute inclusion. In some stones a technical flaw does not detract but adds to the character of the stone, e.g. the inclusions that cause the asterism in a ruby or sapphire, the dendritic inclusions in a moss agate, and the structure of an opal.
A stone is said to be 'clean' or 'flawless' if no flaws are noticeable under a jeweller's lens that magnifies ten times (or 'VVSI' if 'very very slightly imperfect'). Some flaws can be concealed by the manner in which the stone is set.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson