Grain rupin (also known as prismatin ) is a scarce borosilicate material of the formulae (Mg,Fe2+)4(Al,Fe3+)6(SiO4,BO4)5(O,OH)2. Crystallises in the orthorhomboid-di pyramidal crystalline system as amber, verdant, colourless to slim turmaline like prism or in solid fibre shapes. Grain rupin is appreciated as a precious stone when it occurs in tranlucent greens or yellows.
Particularly in demand are the emerging greens. They form a fixed set of solutions with prismatins Strongly pleochroitic, seen from different angles they appear either red or green-brown. The first description was in 1884 for an occurence in Fiskernæs in south-west Greenland. The name Kornerup was given in honour of the famous Dane gemstone ologist Andreas Nikolaus Kornerup (1857-1883) Although Kornerupine was mentioned in 1884, it was not until 1912 that gemstone materials were found, which are still unusual today.
Cornerupine Gemstone Series | Learning Center
Cornerupine was first found in Greenland in 1884 and is called after the name of the famous gemologist Andreas N. Kornerup. Good qualitiy grain rupin, when cabochons are sliced, can show the "cat's eye" effect and sometimes also "asterism" (a stellar effect). The most infrequent color to find a grain rupee is color. The unique feature is that they are strikingly similar to emerald not only in color but also in refraction index.
However, it is characterised by its pleochroitic characteristic, i.e. it is possible to see different colors from different perspectives. The gemstone is not the only conspicuous color to be found in this gemstone, as it is also available in yellow, brown, pink and even in a clear one.
Corn rupins can show colors from yellowish-green to brownish-red when viewed through different corners and facettes. Like many of the more rare gems, grain rupees rates are jumping quasi for bigger items. In Sri Lanka, grain rupins are usually found in placer deposit.
There are also occurrences of Kornerupine in Madagascar and Australia. Beautifully clear for precious stones, if you study the polish from above, it offers an appealing symmetric look and the eye is of course pulled along the line of the pavillon to the central calet.