I know that one of the kanji for Steel is 鋼 and one of the pronunciation is はがね (hagane). But in some mangas I read this kanji with the pronunciation がね (gane) without the は; like 黒鋼 (くろがね/Kurogane).
I would like to know what is the difference between the two of them and if there is any grammar rule that applies in each case.
Thank u very much.
This がね is the rendaku form of かね, which is a wago (native Japanese word) that means "money" or "metal" depending on the context. はがね is etymologically 刃 + かね ("blade metal"). こがね ("yellow metal"), くろがね ("black metal"), あかがね ("red metal") are old words for gold, iron and cupper, respectively.
The default kanji for かね in modern Japanese is of course 金, but as you can see in the link above, several other kanji have been historically assigned to かね, too. くろがね is basically an obsolete word, but you may still see it used in fiction (especially fantasy), and one of the old kanji for かね may be used as well in such cases to add some flavor. So 黒鋼, 黒鉄 and 黒銀 are all possible kanji for くろがね, but you can usually expect furigana when such nonstandard words are intentionally used in fiction. In modern standard Japanese, the only kun-reading of 鋼 is はがね, and it plainly refers to steel (and only steel).
By the way, 白金 is a tricky word; it is normally read as はっきん and refers to platinum ("white gold") in modern Japanese, but it used to be read as しろがね/しろかね and referred to silver ("white metal").
