ENDespite the long since established and generally accepted etymology of the Lith. deimantas 'diamond' as having been borrowed from Polish and, like other European equivalents, originally from Lat. diamafnjs, -antis and Gr. adamas, -antos 'unconquerable', the author dares to suggest another and quite different solution. It rests, firstly, on the Indian origin of diamonds and typologically on the fact that Skr. vajra-, the main word for 'diamond' in this language, originally meant just 'thunderbolt'. Secondly, attention is drawn to the developed mystical symbolism of a thunderbolt as 'enlightenment', particularly the vajra- as 'enlightened mind1, of which both diamond and gems generally partake too. This opens up an opportunity to compare the appellative deimantas (var. deimentas, diemantas, diementas, etc.) with the compound proper name Deimantas, made of the roots Lith. dei- / die- (cf. deivas, dievas 'god', diena 'day') and man(t)- / men(t)- (cf. mantus 'intelligent, ingenious, clever', menta 'mind', manyti ‘think, mean'), derived respectively from the IE. *dei- 'shine brilliantly, brilliant light' and *men- 'think, mean, thought, mind'. A corresponding combination is found in the Rigveda X.177.2.C as dyotamänäm manisäm (sg. acc.) 'brilliant knowledge, or thought, or hymn'. Since Lithuanian Perkūnas, like Indian Indra, is also distinguished as a 'musician', the word deimantas could be conjectured to mean primarily some mytho-poetical reality, for instance, an 'enlightening thunderbolt' of a sacred hymn, or sacred knowledge, and only secondly to acquire the current meaning of a mineral, due to the characteristic features and symbolism of the diamond and the loss of the old Lithuanian (Baltic) spiritual tradition. [From the publication]