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Continue Reading about Gemstone Properties Gemstone Clarity Gemstone Color Gemstone Form Gemstone Scarcity Gemstone Hardness Specific Gravity Luster Gemstone Crystal System |
Cleavage is usually described with reference to crystallographic axes and directions. However, this nomenclature is beyond the scope of this book, so in all cases only the number of cleavage directions in a gem species has been indicated and whether the cleavage is perfect (eminent), good, fair, or poor. Sometimes there are different degrees of cleavage perfection in different directions within the same crystal, and these have been so indicated in the text.
The term parting refers to breakage of minerals along directions of structural weakness. Unlike the situation in cleavage, parting is not present in all specimens of a given species. Fracture is the way a mineral breaks other than along cleavage directions. The descriptive terms for this property are: conchoidal (shell-like, distinguished by |
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concentric curved lines; this is the way glass breaks); fibrous; splintery; hackly (consisting of sharp-edged and jagged fracture surfaces); uneven.
Gems with perfect cleavage must be set carefully and worn carefully, as a sharp blow to the stone along a cleavage direction may easily split the gem. Spodumene is well known for its difficulty in cutting. Even topaz offers occasional problems to the cutter who is not aware of the cleavage direction, because it is virtually impossible to polish a gemstone surface that is parallel to a cleavage plane.
Gems with perfect cleavage must be set carefully and worn carefully, as a sharp blow to the stone along a cleavage direction may easily split the gem. Spodumene is well known for its difficulty in cutting. Even topaz offers occasional problems to the cutter who is not aware of the cleavage direction, because it is virtually impossible to polish a gemstone surface that is parallel to a cleavage plane.
References:
1- Color Encyclopedia of Gemstones, Joel E. Arem, VNR publications, New York.
1- Color Encyclopedia of Gemstones, Joel E. Arem, VNR publications, New York.