- Scratch hardness
Scratch hardness is the measure of how resistant a sample is to fracture or plastic (permanent) deformation due to friction from a sharp object. The principle is that an object made of a hard material will scratch an object made of a softer material. The most common test is Mohs scale, which is used in mineralogy. One tool to make this measurement is the sclerometer.
- Indentation hardness
Indentation hardness measures the resistance of a sample to permanent plastic deformation due to a constant compression load from a sharp object; they are primarily used in engineering and metallurgy fields. Common indentation hardness scales are Rockwell, Vickers, Shore, and Brinell.
- Rebound hardness
Rebound hardness, also known as dynamic hardness, measures the height of the "bounce" of a diamond-tipped hammer dropped from a fixed height onto a material. This type of hardness is related to elasticity. The device used to take this measurement is known as a scleroscope. Two scales that measures rebound hardness are the Leeb rebound hardness test and Bennett hardness scale.
The Mohs scale
The Mohs scale of mineral hardness is based on the ability of one natural sample of matter to scratch another. The samples of matter used by Mohs are all minerals. As the hardest known naturally occurring substance when the scale was designed, diamonds are at the top of the scale. The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, and/or the softest material that can scratch the given material.
The Mohs scale is a purely ordinal scale and hence it just represent the relative hardness of the material and not the absolute values. The table below shows comparison with absolute hardness measured by a sclerometer, with pictorial examples.
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Mohs hardness table. Photo courtesy: Wikipedia |
Hardness of Ruby
The hardness of ruby crystal is attributed to its structure. The crystal has a Hexagonal Scalenohedron structure.
Crystal Family
The crystals in ruby are in the hexagonal class of crystals, giving them four axes of symmetry -- one vertical and three in one plane. It is this plane that gives the class its "hexagonal" name since the three axes are spaced evenly, creating a hexagonal shape. Corundum crystals fall into the subcategory that has lower symmetry, known as trigonal, only having three-fold symmetry around the vertical axis.
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| Hexagonal Crystal System. Photo courtesy: All About Gemstones |
Specific Crystallization Patterns
Officially, rubies display a particular crystal structure known as the hexagonal scalenohedron. A scalenohedron is a twelve-sided polyhedron, and each of its faces is an identical scalene triangle, or a triangle with three different side lengths. These faces are arranged with six triangles, each forming two pyramid-like shapes, albeit with uneven bottom edges. These two pyramidal structures meet along their open edges. This structure as a whole is known in geometry as a hexagonal scalenohedron and so gives its name to the crystallization pattern of corundum.
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| Hexagonal Scalenohedron. Photo courtesy: Gemology Online |
Crystalline Complications
The hexagonal scalenohedron crystal pattern is one of the mineral patterns that most often displays irregularities of form. This is where, instead of forming perfectly regular structures composed entirely of scalenohedrons, the ions will "slip" into other, similar crystalline shapes for short stretches of the mineral. These discontinuities are known as "form combinations" and give naturally-formed rubies the fascinating variety of shapes and faces they display
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Crystalline structure of Ruby. Photo courtesy: eHow |
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