The first fragment (15.91 kilograms (35.1 lb)) was found on 9 May 1984 in a barley field near Twann, after it had been ploughed (Twannberg I). Two additional fragments were found in an unnatural setting. One fragment (II) was found 2000 in an attic in Twann and another (III) in 2005 in the Naturhistorisches Museum Bern, where it had been labeled as hematite. Twannberg IV, V and VI were found in the creek Twannbach.
The meteorite is named after Twannberg (from German: Twann mountain), a mountain that lies north of Twann.
The find location is a glacial till deposit. This pared with minerals found in the oxidized surface of the meteorite are an indicator that the meteorite fell on the Rhone glacier and was transported by it to Twann during the Würm ice age and then deposited there.
The meteorite is composed of meteoric iron and schreibersite. The meteoric iron has 5.1% Nickel, and is all in the form of kamacite. The preatmospheric mass was estimated to be at least 11,000 kilograms (24,000 lb). The surface of the meteorite is covered in an oxidation layer.
^ abcdHofmann, Beda A.; Lorenzetti, Silvio; Eugster, Otto; Krähenbühl, Urs; Herzog, Gregory; Serefiddin, Feride; Gnos, Edwin; Eggimann, Manuel; Wasson, John T. (1 February 2009). "The Twannberg (Switzerland) IIG iron meteorites: Mineralogy, chemistry, and CRE ages". Meteoritics & Planetary Science44 (2): 187–199. doi:10.1111/j.1945-5100.2009.tb00727.x.Cite uses deprecated parameters (help)
^Wasson, John T.; Choe, Won-Hie (31 July 2009). "The IIG iron meteorites: Probable formation in the IIAB core". Geochimica et Cosmochimica Acta73 (16): 4879–4890. doi:10.1016/j.gca.2009.05.062.Cite uses deprecated parameters (help)