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Gemstone Properties:   Achroite   Actinolite   Alexandrite   Almandine   Amber   Amethyst   Ametrine   Andradite   Andalusite   Apache Tear   Apatite   Aquamarine   Axinite   Barite   Benitoite   Beryl   Bixbite   Bytownite   Calcite   Cassiterite   Cerussite   Chrysoberyl   Citrine   Clinozoisite   Cordierite    Corundum   Cubic Zirconia   Danburite   Demantoid   Diamond   Diopside   Dravite   Ekanite   Elbaite   Emerald   Epidote   Euclase   Feldspar   Fluorite   GGG   Garnet   Goshenite   Gypsum   Hauynite   Heliodor   Hiddenite   Indicolite   Idocrase   Iolite   Kornerupine   Kunzite   Kyanite   Labradorite   Moldavite   Morganite   Obsidian   Opal   Orthoclase   Padparadschah   Peridot   Petalite   Piemontite   Phenacite   Prasiolite   Quartz   Rhodochrosite   Rhodolite   Rubellite   Ruby   Sapphire   Scapolite   Scheelite   Selenite   Sinhalite   Sphalerite   Sphene   Spinel   Spodumene   Schorl   Spessartite   Sunstone   Tanzanite   Topaz   Topazolite   Tourmaline   Tektite   Tsavorite   Verdelite   Vesuvianite   Wulfenite   YAG   Zincite   Zircon   Zoisite  

Corundum (aluminum oxide)

Types/Color:

  • Synthetic Corundum – Synthetic is usually called corundum and is available in many colors.

  • Padparadshah – Strong yellow, orange-pink to hot pink.

  • Ruby – Red to dark pink. Usually has a hint of violet.

  • Sapphire – Natural corundum is commonly thought of as dark blue, but it occurs in many other colors such as yellow, green, pink, colorless and purple.

Moh’s hardness: 9

Refractive index: 1.766-1.774

Critical angle: 34.5°

Specific gravity: 3.99-4.0

Cleavage: None

Fracture: Uneven, conchoidal, splintery.

Dispersion: .Moderately high (018)

Heat sensitivity: None

Pleochroism: Dichroic.  Darker colors usually show stronger dichroism.  Natural blue: distinct (dark blue to greenish-blue); Natural green: weak (more or less yellow); Natural orange: strong (orange to yellow-brown or colorless); Natural purple: distinct (purple to pinkish); Natural ruby: strong (deep red to yellowish-red); Natural yellow: weak (yellow to light-yellow).

Birefringence (double refraction): Weak (.018)

Crystal structure: Hexagonal (trigonal), dipyramidal

Treatments: Heat treating to reduce silk is standard.  Glass-filling voids in rubies is not considered an acceptable treatment.

Cutting: Corundum cuts and polishes at varying rates on different facets.  Don’t cut with a bonded lap at any finer grit than 600, otherwise an orange peel effect will make polishing a nightmare.  Pre-polish with 8,000 grit on ceramic, zinc or copper lap helps speed up final polishing immensely.

Polishing: Ceramic with 100,000 or 200,000 diamond spray, or zinc lap with 50,000 or 100,000 diamond.  BATT can be used as a polish lap, but is a little soft for corundum.

Comments: Orient for best color and yield.  Corundum cuts and polishes differently on different facets.

 

Gemstone Properties:   Achroite   Actinolite   Alexandrite   Almandine   Amber   Amethyst   Ametrine   Andradite   Andalusite   Apache Tear   Apatite   Aquamarine   Axinite   Barite   Benitoite   Beryl   Bixbite   Bytownite   Calcite   Cassiterite   Cerussite   Chrysoberyl   Citrine   Clinozoisite   Cordierite    Corundum   Cubic Zirconia   Danburite   Demantoid   Diamond   Diopside   Dravite   Ekanite   Elbaite   Emerald   Epidote   Euclase   Feldspar   Fluorite   GGG   Garnet   Goshenite   Gypsum   Hauynite   Heliodor   Hiddenite   Indicolite   Idocrase   Iolite   Kornerupine   Kunzite   Kyanite   Labradorite   Moldavite   Morganite   Obsidian   Opal   Orthoclase   Padparadschah   Peridot   Petalite   Piemontite   Phenacite   Prasiolite   Quartz   Rhodochrosite   Rhodolite   Rubellite   Ruby   Sapphire   Scapolite   Scheelite   Selenite   Sinhalite   Sphalerite   Sphene   Spinel   Spodumene   Schorl   Spessartite   Sunstone   Tanzanite   Topaz   Topazolite   Tourmaline   Tektite   Tsavorite   Verdelite   Vesuvianite   Wulfenite   YAG   Zincite   Zircon   Zoisite  
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