Corundum

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Definition

The mineralogical name for Al2O3, the corundum family encompasses the gem varieties sapphire and ruby. It crystallizes in the trigonal structure and is surpassed only by diamond in hardness.

Etymology

Whether the word 'corundum' originates from the hindu 'kurand', 'kuruvinda' or 'Kauruntaka' isn't clear but that the word has it's origin in the hindu language appears to be clear.

History

Sapphires2.jpg
The fact that sapphire and ruby were one and the same mineral with merely their colors differing had probably been known by their miners in Sri Lanka for ages. In Europe this wasn't so. From Theophrastus' classical work to de Boodt's writings in 1609 there is no proof to be found of anyone realizing the two were one and the same mineral. In 1652 Thomas Nichols wrote a lapidary in which he hints at a relation between the two family members but doesn't make the call. It was in 1796 when René Hauy described ruby and sapphire as single species and over the years after that a series of papers was published, finally uniting all corundum varieties under the single name in 1805.

Crystallography & Properties

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Ruby2.JPG
Corundum crystallizes in the trigonal structure and is most often found as tabular hexagonal crystals (ruby) and hexagonal bipyramids (sapphire) although other habits may be encountered as well. Twinning is quite common. Corundum is aluminum oxide and, when pure, colorless. Color in corundum is caused by impurities, the most common being chromium which causes the red in ruby and iron with titanium which is responsible for the blue in sapphire.

As said, only diamond is harder than corundum. The mineral species has been describes as having no cleavage throughout history but Richard W. Hughes mentions Russian research in his book Ruby & Sapphire that suggests cleavage to exist in corundum. This was in synthetic samples of extreme purity though, it has not been experienced by lapidaries working with corundum on a day to day basis. The world's second hardest mineral does occasionally part along certain planes due to the exsolution of boehmite or hematite-ilmenite. The exsolved impurities cause structural weakening along the plane they exsolve in.

For further specific data on corundum see the ruby and sapphire pages.

Corundum facts
Crystal Structure Trigonal
Hardness 9
Fracture Conchoidal
Refractive Index 1.762-1.770
Durability Very durable

Online G&G articles on corundum

Lang Antiques
Lang Antiques