…the PIE word for 'stone' secondarily refers to 'heaven' in Indo-Iranian and Germanic; while we are not entirely certain of the underlying association, it may rest on a conception of the heaven as a stony vault, from which fragments might fall in the form of meteorites; or it may be connected with the stony missiles thought to be hurled by the god of thunder.
The Proto-Indo-European root is *h₂eḱ-men- "stone", a suffixed form of *h₂eḱ- "sharp". In Proto-Germanic, *h₂eḱ-men- metathesized to something like *ke-men- then *himin, dissimilated to *hiƀin-. This became Old English heofon, then heaven. Presumably German Himmel is from the undissimilated form.
The Indo-Iranian word that Fortson mentions is Sanskrit aśman- "stone, heaven", and Persian āsmān "heaven". It's worth noting that, according to the OED, the connection between the Indo-Iranian and Germanic words is "rejected by many" on both semantic and phonological grounds.
A suffixed form of *h₂eḱ-, *h₂eḱ-ri-, became Latin ācer "sharp, bitter", becoming Old French aigre "sour", which combined with vin "wine" to form vinaigre and English vinegar.