Corundum is a mineral composed of aluminum oxide. Living deep within the Earth in an environment of intense heat and pressure, the material undergoes transformation over millions of years. It's moved by glaciers and tectonic action.
Corundum forms in diverse geological settings. It's found in metamorphic rocks such as schist or gneiss, alluvial deposits and gem-bearing gravels. Forged with fire and force, corundum is a very hard, tough and stable mineral. It's unaffected by acids and most environments.
The name corundum is first derived either from the Sanskrit word Kuirvinda or from the Indian name for corundum, Kauruntoka. A crystalline form of aluminum oxide (Al2O3), corundum is considered one of the hardest natural substances on Earth.
It ranks 9 on the Mohs scale of mineral hardness. Comparatively, diamond is 10, and glass is 5.5 - 6. Corundum's composition creates many structural formations, resulting in different varieties, most notably sapphire and ruby.
A crystalline form of aluminum oxide, corundum typically contains traces of iron, titanium, vanadium and chromium. A rock-forming mineral, it's naturally transparent and can have different colors depending on the type of transition metal inclusions in its crystalline structure.
Sapphires can be any color but red, while rubies can be varying shades of red from clear and bright to deep and rich. Purple-red and vividly pink rubies are sometimes found.
Rubies are red due to presence of the metal chromium, and sapphires exhibit a range of colors depending on what transition metal is present. A rare type of sapphire, padparadscha sapphire, is pink-orange. Padparadscha is from the Sinhalese for "aquatic lotus blossom."
History and Geology
Knowledge and use of corundum dates back thousands of years. Ancient civilizations admire sapphires and rubies for brilliance and colors. Sapphires are reported in rings from 800 BCE. Ancient Sanskrit texts refer to rubies.
The belief in the mystical powers of rubies resulted in their use in regal ornamentation, representing power, passion, and protection. Ruby and sapphire are social and religious status symbols up to the present day.
Geologically, corundum plays a crucial role in studying the Earth's crust and its formation, giving insight into conditions under which it forms. Corundum in rock sequences can indicate high-pressure environments, often connected to mountain-building and strong tectonic activity.
Corundum forms under high-temperature and high-pressure conditions, commonly found in metamorphic and igneous rocks. The primary sources of corundum include:
Metamorphic Rocks: Corundum often develops within metamorphosed rocks where aluminum-rich minerals, such as kyanite, garnet, or spinel, decompose and dissolve under intense pressure and heat.
Igneous Rocks: It can also crystallize from molten rock (magma) when conditions permit. In this context, corundum can form in pegmatitic deposits, which are characterized by large crystals.
Alluvial Deposits: Erosion of corundum-rich rocks can lead to alluvial deposits, where the mineral is sorted and concentrated by the action of water.
Corundum is picky. It requires specific conditions for crystallization, making it a scarce mineral, particularly in gem-quality form. Workers at alluvial sites might labor for weeks without finding a corundum gem.
Properties of Corundum
Corundum's enticing beauty is complemented by its remarkable physical and chemical properties. Key features:
Hardness: Ranked 9 on the Mohs hardness scale, corundum is nearly impervious to scratches, a quality highly sought after in jewelry.
Color: Pure corundum is colorless, but traces of other elements impart characteristic colors. For example, chromium gives rubies their red hue, while iron and titanium can create blue sapphires. The presence of different impurities leads to a range of colors, including yellow, green, orange, and pink.
Luster: Corundum exhibits a vitreous to adamantine luster, providing a brilliant shine when polished. This luster enhances its appeal in jewelry.
Transparency: While some corundum forms are opaque, gem-quality corundum crystals are usually transparent to translucent.
Crystal Structure: Corundum is crystallized in the hexagonal system, forming prismatic ones, often associated with rhombohedra or hexagonal bipyramids.
Corundum, with its striking beauty and formidable hardness, is a gem of geological, historical and economic significance. Knowing its formation, context of its existence in our planet’s crust, and defining characteristics give insight into not only geology and cultural history.
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