Corundum
Chemical formula: Al₂O₃
Corundum is an aluminum oxide, known for its exceptional hardness, forming the basis of many valuable gemstones such as ruby and sapphire.
Description
## Characteristics Corundum is a mineral belonging to the oxide group, chemically being aluminum oxide (Al₂O₃). It occurs as prismatic, tabular, or barrel-shaped crystals, often with a hexagonal outline. Its surfaces can be striated. It also frequently forms granular or massive aggregates. Pure corundum is colorless, but trace element impurities give it a wide range of colors, making it one of the most color-diverse minerals. ## Physical Properties Corundum is characterized by very high hardness, measuring 9 on the Mohs scale, making it the second hardest mineral after diamond. It has a vitreous to adamantine luster. It is brittle. The density of corundum is approximately 3.98 g/cm³. ## Colors and Varieties Corundum occurs in many colors, depending on the presence of impurities. The most famous varieties are red ruby (owing its color to chromium) and sapphire, which can take on blue (iron and titanium), yellow, pink, green, violet, and even orange (padparadscha) hues. Colorless, gray, and brown varieties also exist. ## History and Name The name "corundum" likely originates from the Sanskrit word "kuruvinda," meaning "ruby." This mineral has been known and valued since antiquity, especially its noble varieties. Ruby and sapphire have a long history of use in jewelry and as symbols of power and wealth. ## Applications Due to its exceptional hardness, corundum is widely used as an abrasive material in industry, for the production of sandpaper, grinding wheels, and polishing pastes. The noble varieties – ruby and sapphire – are among the most valuable gemstones. Synthetic corundum is used in watchmaking (crystals, bearings), in lasers, and as a substrate in electronics.
Diagnostic features
## Identification Corundum can be identified primarily by its exceptional hardness (9 on the Mohs scale), which allows it to scratch most other minerals, except diamond. Its hexagonal crystals, vitreous to adamantine luster, and high density are also characteristic. For colored varieties, the intensity and uniformity of color are important diagnostic features. ## Distinguishing from Similar Minerals Corundum is sometimes confused with garnet, spinel, topaz, or tourmaline. It is distinguished from garnet and spinel by its hardness (corundum is harder) and crystallographic system. It is also distinguished from topaz and tourmaline by its higher hardness. The hardness test is crucial in distinguishing corundum from similar minerals. ## Crystal Forms Corundum most often forms prismatic, tabular, or barrel-shaped crystals, often with a distinct hexagonal outline. Crystal surfaces can be striated. It also occurs as granular or massive aggregates, as well as rounded grains in alluvial deposits.
Geological environment
## Genesis Corundum forms in various geological environments. It occurs in igneous rocks, such as nepheline syenites and pegmatites, where it crystallizes from magma rich in aluminum and poor in silica. It is also found in metamorphic rocks, especially in marbles, gneisses, and schists, where it forms as a result of contact or regional metamorphism. It often occurs secondarily in alluvial placer deposits, where it is resistant to weathering due to its hardness and density. ## Mineral Associations Corundum often co-occurs with minerals such as spinel, garnet, zircon, rutile, kyanite, sillimanite, andalusite, muscovite, biotite, plagioclase, and magnetite. ## Localities Known deposits of corundum, including its noble varieties, are found in many places around the world. Important localities include Sri Lanka, Burma (Myanmar), Thailand, Vietnam, Cambodia, India, Madagascar, Tanzania, Kenya, Australia, the United States (Montana, North Carolina), and Russia (Urals).
Rarity
Very Common
Collector aspects
## Quality Criteria The most important quality criteria for corundum specimens are color, transparency, clarity (lack of inclusions and fractures), size, and crystal perfection. For rubies and sapphires, color intensity and saturation are crucial. Specimens with well-formed crystals and rare colors are particularly valued. ## Popular Localities The most prized ruby specimens come from Burma (Myanmar), especially from the Mogok Valley. Sapphires of the highest quality, including the famous Kashmir sapphires, originate from India. Other important sources include Sri Lanka (ceylonites), Thailand, Madagascar, and Tanzania.
Care and storage
## Cleaning Corundum, due to its hardness, is relatively resistant to mechanical damage. For cleaning most specimens, warm water with mild soap and a soft brush is sufficient. For specimens with complex structures or inclusions, distilled water can be used. ## What to Avoid Avoid cleaning corundum with strong chemicals, especially acids, which can damage some inclusions or the surface. Although corundum is resistant to high temperatures, sudden thermal changes can lead to cracks, especially in specimens with inclusions. Prolonged exposure to intense sunlight does not negatively affect the mineral itself, but it can be harmful to some colored varieties, causing them to fade. Ultrasonic cleaners should also be avoided for specimens with cracks or inclusions. ## Storage Corundum should be stored separately from other, less hard minerals to prevent scratching them. Soft pouches, separate boxes, or display cases are ideal for this. Stable conditions should be ensured, avoiding large fluctuations in temperature and humidity.