Chemical composition: C, carbon
Hardness: 10 Mohs' scale, 56-115 Knoop hardness number (GPa), 10,000 Brooks identer scale.
Crystallography: Isometric. Crystals sometimes sharp octahedral, rhombic dodecahedral, cubes, twinning, plates, and combinations with other forms. Crystals modified often rounded and distinguished by the presents of triangular shaped pits on the faces of the octahedral shaped crystals. These trigons were once thought to be the result of etching are now believed to be part of the growth process.
Luster: Adamantine (diamond provides the definition for this kind of luster).
Cleavage: Excellent, parallel to octahedron face, four directions.
Density: 3.51 g/cm3, or specific gravity = 3.51 (3.51at 3.53).
Refractive Index: 2.4175 in the yellow light of a sodium lamp. (2.417 at 2.419)
Dispersion: Large (0.044), leading to rainbow colors on refraction.
Optics: Isotropic. Index very constant. N =2.417.
Thermal Conductivity: Superb. 5-25 Watts/centimeter-degreesC (at 300 K). 4 times greater than copper, an excellent thermal conductor.
Electrical Conductivity: 0 to ~ 100 ohm-cm (resistivity at 300 K), an insulator.
Optical Transmission: Transparent over broad spectrum of the electromagnetic spectrum. An excellent material for optical windows.
Index of refraction: the index of refraction is defined as the speed of light in vacuum divided by the speed of light in the medium.
Birefringence: a single light ray is split into two rays as it is transmitted into uniaxial and biaxial minerals. The two rays travel at different velocities through the mineral, which is why uniaxial minerals have two indices of refraction (nw and ne). The difference between the two indices of refraction is called birefringence.
Pleochroism: the splitting of light into two light rays may result in an effect called pleochroism in some colored transparent uniaxial and biaxial minerals. Pleochroism is produced because the two rays of light are differencially absorbed as they pass through the colored anisotropic mineral. Because the absorption is different for the two light rays, the wavelengths and colors of the two light rays are different when they emerge from the crystal. The color of ruby and sapphire (uniaxial corundum) is caused in part by a blending of the two colors. Biaxial minerals, or those that crystallize in the orthorhombic, monoclinic, and triclinic crystal systems, can have three distinct colors. Tiny crystals of biaxial ivaite, the black mineral in the photograph to the left, is strongly pleochroic (yellow, dark brown, and dark green).
Dispersion: the index of refraction of an isometric mineral is not the same for all wavelengths of light. When a ray of white light travels from air into a triangular glass prism, the light not only bends but it also is separated into its component colors or wavelengths, the colors of the spectrum. The violet light is bent slightly more than the red, for instance, because it travels more slowly through the glass. As the light emerges from the prism the colors separate even more. This phenomenon, called dispersion, can be observed in some minerals such as diamond. Jewelers refer to this characteristic in gemstones as fire. The next time you look at a diamond, observe the rainbow of colors caused by the splitting of white light into its component wavelengths.
For more information on gemstones, we invite you to visit some sites:
Gem Hut: serving the Internet with fine and rare colored gemstones from around the world.
Bob's Rock Shop: is a mineral 'zine with many links to other rockhound or mineral topics.
Mineralogical Association of Canada: in addition to regular bi-monthly articles on minerals, publishes the IMA listing of approved minerals in their journal, "The Canadian Mineralogist." You can browse the abstracts on-line.
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