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Ukonvasara, or Ukonkirves, is the symbol and magical weapon of the Finnish thunder god Ukko, similar to Thor's Mjölnir. Ukonvasara means hammer of Ukko; similarly, Ukonkirves means axe of Ukko. It was said that Ukko created lightning with Ukonvasara.
Ukko's hammer was probably originally a boat-shaped stone axe. When stone tools were abandoned with the advent of metalworking, the origins of stone weapons became a mystery. Stone axes, so-called thunderstones (ukonvaaja in Finnish), were found in the ground, especially after drenching rains washed away dirt. They were believed to be weapons of Ukko, stone heads of the striking lightning. Shamans collected and held stone-axes because they were believed to hold the power to both heal and damage.
According to Asko Parpola, the Sanskrit vajra- and Avestan vazra- both refer to a weapon of the Godhead, and are possibly from the Proto-Indo-European root *weg'- which means "to be(come) powerful". It is related to Proto-Finno-Uralic *vaśara, "hammer, axe", but both the Sanskrit and Finno-Ugric derivatives are likely Proto-Aryan or Proto-Indo-Aryan but not Proto-Iranian, state Parpola and Carpelan, because of its palatalized sibilant.
Unto Salo believes that Ilmari, another Finnic sky god, is the origin of Ukko, but that as Ukko Ilmari experienced very significant, although far from total, influence from the Indo-European sky god especially in the form of Thor. Others believe that Ukko's original name was Baltic Perkūnas.
Perkūnas is pictured as middle-aged, armed with an axe and arrows, riding a two-wheeled chariot harnessed with goats, like Thor The name Thor descends from the Proto-Germanic theonym *Þun(a)raz ('Thunder'), According to scholar Peter Jackson, those theonyms may have originally emerged as the result of the fossilization of an original epithet (or epiclesis) of the Proto-Indo-European thunder-god *Perkwunos. from which Perkunas also descended from 
Indra is described as using the vajra to kill sinners and ignorant persons. Indra's mythology parallels Perun, Perkūnas, Taranis, and Thor, suggesting a common origin in Proto-Indo-European mythology.
- Parpola & Carpelan 2005, p. 118. sfn error: no target: CITEREFParpolaCarpelan2005 (help)
- Asko Parpola 2015, pp. 63–66, 114. sfn error: no target: CITEREFAsko_Parpola2015 (help)
- Douglas Q. Adams (1997). Encyclopedia of Indo-European Culture. Routledge. p. 112. ISBN 978-1-884964-98-5.
- Salo, Unto (1990). Agricola's Ukko in the light of archeology. A chronological and interpretative study of ancient Finnish religion: Old Norse and Finnish religions and cultic place-names. Turku. ISBN 951-649-695-4.
- Salo, Unto (2006). Ukko: The God of Thunder of the Ancient Finns And His Indo-european Family. Inst for the Study of Man. ISBN 978-0941694940.
- Siikala, Anna-Leena (2013). Itämerensuomalaisten mytologia. Helsinki: SKS.
- "Gintaras Beresnevičius, Lithuanian Mythology". Crvp.org. Archived from the original on 2012-09-02. Retrieved 2012-09-03.
- Orel 2003, p. 429 harvnb error: no target: CITEREFOrel2003 (help), Delamarre 2003, p. 290 harvnb error: no target: CITEREFDelamarre2003 (help)
- Jackson, Peter (2002). "Light from Distant Asterisks. Towards a Description of the Indo-European Religious Heritage". Numen. 49 (1): 61–102. doi:10.1163/15685270252772777. ISSN 0029-5973. JSTOR 3270472.
- Dowden, Mr Ken; Dowden, Ken (4 January 2002). European Paganism: The Realities of Cult from Antiquity to the Middle Ages - Mr Ken Dowden - Google Böcker. ISBN 9780203011775. Retrieved 2012-09-03.
- Rigveda 2.12
- Thomas Berry (1996). Religions of India: Hinduism, Yoga, Buddhism. Columbia University Press. pp. 20–21. ISBN 978-0-231-10781-5.
- T. N. Madan (2003). The Hinduism Omnibus. Oxford University Press. p. 81. ISBN 978-0-19-566411-9.
- Sukumari Bhattacharji (2015). The Indian Theogony. Cambridge University Press. pp. 280–281.