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Rubies and sapphires
'''Corundum''' is the second hardest natural mineral known to science. The hardest mineral, [[diamond]] is still four times harder than corundum. The hardness of corundum can be partially attributed to the strong and short oxygen-aluminum bonds. These bonds pull the oxygen and aluminum atoms close together, making the crystal not only hard but also quite dense for a mineral made up of two relatively light elements.
PHYSICAL CHARACTERISTICS:
* Color is highly variable. The color can be white or colorless, blue, red, yellow, green, brown, purple, and pink; there are also instance of color zonation.
* Transparency: Crystals are transparent to translucent.
* Crystal System is trigonal
* Crystal Habits include sapphire's typical six-sided barrel shape that may taper into a pyramid, and ruby's hexagonal prisms and blades.
* Hardness is 9
* Specific Gravity is approximately 4.0+ (above average for translucent minerals)
* Associated Minerals include calcite, zoisite, feldspars, micas and garnets.
* Other Characteristics: Refractive index is around 1.77; pleochroic (meaning color intensity is variable from different viewing directions); striations on parting surfaces.
* Notable Occurrences include Burma; Sri Lanka; North Carolina and Montana, USA; many African localities; several localities in India, and Middle Eastern and Southeast Asian countries.
* Best Field Indicators are extreme hardness, density and crystal habit.<ref>[http://www.galleries.com/minerals/oxides/corundum/corundum.htm]</ref>
<references/>
[[category:Geology]]
[[Category:Minerals]]
[[Category:gems]]
PHYSICAL CHARACTERISTICS:
* Color is highly variable. The color can be white or colorless, blue, red, yellow, green, brown, purple, and pink; there are also instance of color zonation.
* Transparency: Crystals are transparent to translucent.
* Crystal System is trigonal
* Crystal Habits include sapphire's typical six-sided barrel shape that may taper into a pyramid, and ruby's hexagonal prisms and blades.
* Hardness is 9
* Specific Gravity is approximately 4.0+ (above average for translucent minerals)
* Associated Minerals include calcite, zoisite, feldspars, micas and garnets.
* Other Characteristics: Refractive index is around 1.77; pleochroic (meaning color intensity is variable from different viewing directions); striations on parting surfaces.
* Notable Occurrences include Burma; Sri Lanka; North Carolina and Montana, USA; many African localities; several localities in India, and Middle Eastern and Southeast Asian countries.
* Best Field Indicators are extreme hardness, density and crystal habit.<ref>[http://www.galleries.com/minerals/oxides/corundum/corundum.htm]</ref>
<references/>
[[category:Geology]]
[[Category:Minerals]]
[[Category:gems]]