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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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