Kornerupine

Chemical formula: (Mg,Fe²⁺,Al,&#₉₇₂₃;)₁₀(Si,Al,B)₅O₂₁(OH,F)₂

Kornerupine is a rare, transparent magnesium and aluminum silicate, valued in jewelry for its pleochroism and attractive colors, mainly green and brown.

## Characteristics Kornerupine is a complex borosilicate of magnesium, iron, and aluminum, belonging to the group of rare minerals. It forms elongated, columnar, or acicular crystals with a pseudohexagonal cross-section, often terminated by pyramids. Gem-quality specimens are typically transparent and exhibit intense, attractive coloration. A characteristic feature of kornerupine is its strong pleochroism, meaning a change in color depending on the viewing direction, most commonly from green through yellowish to reddish-brown. It also occurs in radial or fibrous aggregates. ## Physical Properties This mineral is characterized by significant hardness, ranging from 6.5-7 on the Mohs scale, which makes it durable and scratch-resistant. The luster is vitreous, and transparency ranges from transparent in gem-quality material to translucent or nearly opaque in aggregates. The density is approximately 3.27-3.45 g/cm³. ## Colors and Varieties Kornerupine occurs in a wide range of colors, including various shades of green (from emerald to olive), brown, yellow, and more rarely it is colorless, blue, pink, or black. The most prized specimens are those with an intense, emerald-green color, often originating from Tanzania and Sri Lanka. Some specimens exhibit a cat's eye effect (asterism), caused by the presence of needle-like rutile inclusions. Vanadium-rich varieties can have an intense blue color. ## History and Name The mineral's name honors the Danish geologist and explorer Andreas Nikolaj Kornerup (1857-1881). Kornerupine was first described in 1884 based on specimens found in Fiskenæsset, Greenland. ## Uses Due to its rarity, attractive colors, and high hardness, transparent varieties of kornerupine are valued gemstones used in jewelry manufacturing. Lower quality specimens and well-formed crystals are sought after by mineral collectors.

Properties

Mohs hardness: 6-7
Color: Colorless, white, blue, green, yellow-brown, black
Luster: Vitreous
Streak: White
Density: 3.29
Cleavage: Good on {110}
Fracture: Uneven to conchoidal
Transparency: Transparent,Translucent,Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification A key diagnostic feature of kornerupine is its strong pleochroism, visible even to the naked eye as a change in hues (e.g., from green to brown) when rotating the crystal. The columnar habit of crystals with a pseudohexagonal outline and vitreous luster are also characteristic. Its hardness (6.5-7) helps distinguish it from many softer minerals. ## Distinguishing from Similar Minerals Kornerupine can be confused with tourmaline, andalusite, enstatite, or sapphire. It is distinguished from tourmaline by a different crystal habit and slightly different optical properties. Andalusite also exhibits strong pleochroism, but usually in different color sets (red, green, yellow). Enstatite is generally less transparent and has different cleavage angles. It is distinguished from sapphire by lower hardness and density. ## Crystal Forms Kornerupine crystallizes in the orthorhombic system, forming elongated, prismatic crystals. It often occurs as radial or fibrous aggregates. Crystals can be twinned, and striations parallel to the crystal's elongation are visible on their faces.

Geological environment

## Genesis Kornerupine is a metamorphic mineral, forming under conditions of high temperature and pressure. It develops in boron-rich sedimentary rocks (e.g., anorthosites, gneisses, granulites) that have undergone intense transformation in the amphibolite and granulite facies. It can also occur in skarns and volcanic xenoliths. ## Mineral Associations This mineral often co-occurs with minerals such as sapphirine, cordierite, sillimanite, andalusite, corundum, garnets (mainly pyrope), biotite, phlogopite, tourmaline, rutile, spinel, and plagioclase. ## Localities The most important localities supplying gem-quality material are Sri Lanka (Ratnapura area), Tanzania (Umba River Valley, Lelatema Hills), Kenya, and Madagascar. Good quality crystals also come from Myanmar (Mogok) and Canada (Quebec). Other known localities include Greenland (Fiskenæsset - discovery site), Norway (Arendal), Australia (Harts Range), and South Africa (Namaqualand).

Rarity

Rare

For collectors

## Quality Criteria The most highly valued kornerupine specimens are those of gem quality – transparent, without visible inclusions or fractures. Color is key: the most desired are intense shades of green (emerald, vanadium-rich) and rare blue and pink colors. Strong, distinct pleochroism increases the stone's value. For opaque specimens, form matters – well-formed, large crystals or aesthetic, radial aggregates are more valuable. ## Market Prices Prices for faceted kornerupines range from tens to several hundred dollars per carat, depending on quality. Stones weighing over 5 carats are rare and can fetch significantly higher prices. The most expensive are clean, intensely green or blue specimens, whose price can exceed 1000 USD per carat. ## Popular Localities Collectors and jewelers most seek specimens from Sri Lanka and Tanzania, which are renowned for producing material of the best color and clarity. Specimens from Madagascar are also very popular. Classic, though less commonly available on the market, are crystals from Greenland.

Care and storage

## Cleaning Kornerupine should be cleaned using lukewarm soapy water and a soft brush. After washing, the stone should be thoroughly rinsed and dried with a soft cloth. Ultrasonic cleaners can be used, but only for specimens without fractures or inclusions. ## What to Avoid Avoid contact with strong acids, especially hydrofluoric acid, which can damage the mineral. Despite its high hardness, kornerupine is brittle and susceptible to mechanical impact. It should not be exposed to sudden temperature changes. ## Storage Collector specimens and kornerupine jewelry are best stored in separate, padded boxes or pouches to prevent scratching by harder stones (e.g., diamonds, corundums) and damage to softer minerals.