In reply to:

Most interesting to me was Quinlon's 'right as an adamant'
(thank you, Bingley, for the link), adamant being at the time a "lodestone or magnet". Nowadays, if we speak adamantly, it means we stick tightly to what we are saying!
Cool!

Whereas when I read that phrase, my mind leapt to diamonds, as I still like the word adamant for diamond. The OED reference to the phrase right as rain to being now chiefly Australian and New Zealand colloquial would explain why my relatives in those countries use the phrase so liberally.