Kornerupine is a borosilicate of magnesium and aluminium, ideally Mg3Al6(Si,Al,B)5O21(OH), occurring as a relatively rare gemstone of distinctly collector character. It was first described in 1884 from the Fiskenaesset region of Greenland and named for the Danish geologist Andreas Kornerup, who had explored Greenland in the 1870s. The species is known to mineralogists as the magnesium-dominant member of a small group, with prismatine being the related boron-rich species (formerly considered a separate mineral, now a defined member of the kornerupine group). For the gem trade kornerupine is significant principally as a source of strongly pleochroic green-to-brown faceted stones, and, more rarely, as the host of a fine cat's-eye chatoyancy.

Physical and optical properties

Kornerupine crystallises in the orthorhombic system, typically in long prismatic crystals striated parallel to the length. The specific gravity falls in the range 3.27 to 3.45 and the hardness is 6.5 to 7 on the Mohs scale, comparable to peridot. The refractive indices are high for a gemstone of its hardness, around 1.667 to 1.689, with a biaxial-negative optic character and a moderate to strong birefringence of about 0.013. The strong pleochroism is the species's diagnostic optical feature: a stone that appears bright green from one direction may appear yellow-brown from another, and the most prized faceted material is cut to balance these axes. Inclusions are typically fluid feathers, growth tubes, and the occasional rutile or other mineral inclusion.

Colour range

The colour of gem kornerupine ranges from sea green through olive and brownish green to yellow and greenish brown. A small proportion of material from East African sources shows a more saturated 'forest' or 'leaf' green that competes visually, at much lower price levels, with fine peridot or chrome tourmaline. Vanadium and chromium have been implicated in the colour of some of the better greens, with iron contributing to the brownish modifier. A blue variety has been reported from Sri Lanka and East Africa but is exceptionally rare. Cat's-eye material is typically greenish brown.

Localities

The principal commercial sources of gem kornerupine have been Sri Lanka (alluvial gravels), Madagascar (the Andilamena area, among others), Tanzania (notably the Umba Valley), Kenya, and Myanmar. Mineralogical type material is from Greenland; gem-quality material is not produced from Greenland in any commercial sense. Material from East Africa, particularly the green chrome-vanadium-bearing stones, has been the most important source for the contemporary connoisseur market, and is the material most frequently encountered in laboratory reports.

Identification

Kornerupine is identified gemmologically by the combination of refractive index, specific gravity, biaxial optic character, strong pleochroism and characteristic absorption spectrum. The species has been confused historically with green tourmaline and with peridot, but its biaxial character (tourmaline is uniaxial; peridot is biaxial but with different RI and birefringence) and its absorption spectrum (a band at around 503 nanometres in the green is diagnostic in many specimens) usually permit confident separation. Modern Raman and FTIR confirmation is straightforward in a well-equipped laboratory.

Position in the market

Kornerupine occupies a clearly collector-grade position in the gem market. It is rarely encountered in mainstream retail jewellery and seldom set in commercial pieces. Its appeal is to gemmologists, mineral specimen collectors who also enjoy faceted material, and customers building specialist pleochroic-stone collections (alongside andalusite, sphene, alexandrite and zoisite). Faceted stones over five carats are uncommon, and clean stones over ten carats are rare. The market has remained small and stable, with prices reflecting the rarity of saturated colour rather than aggressive demand. For the working jeweller it is a useful entry to a connoisseur conversation, particularly when paired with a discussion of pleochroism and orientation in cutting.

Treatment

The gem-trade default for kornerupine is no treatment. The material does not respond meaningfully to heat in the way that aquamarine or zircon do, and no commercial treatment regime has emerged. Buyers should expect natural untreated material, with documentation of any treatment a notable departure from the norm and one to be examined carefully. Cabochon cat's-eye material is, similarly, treated only by cutting and polishing.