ragione.

From Latin ratiōnem, whence also razione (a borrowed doublet). The natural outcome would have been razzone [rat'tsone] (which is attested in some archaic works in Roman vernacular); the reason why the modern form shows a g is that Tuscan literates mistook Northern rason as an outcome of non-existent Latin **rasione~**raseone—whose -sj- indeed evolves regularly into tuscan [ʃ]~[ʒ] and non-tuscan [s]<s> (compare fagiolo from Latin phaseolum, and compare the pronunciation of English vision as "vizhun"—indeed, approvvigionare ultimately comes from Latin visionem with the same assimilation of -sj- into a palatal voiced fricative). Compare other terms ending in -gione. Furthermore, the reason why it’s nowadays pronounced with an affricate rather than a fricative (together with all Italian terms having as the corresponding etymological clusters being -tj- or -sj-, or even -cj- through Gallic languages—see crogiolo) is due to hyper-correction of Tuscan gorgia, i.e. the merging of ungeminated fricatives and affricates into the first ones. Indeed, older spellings with sci or sgi tried to reproduce the ungeminated fricative before the gorgia took place (once it happened, it was more convenient, with merged sounds, to merge the spellings, despite being unetymological).