At least according to Fowler, adamant, as an adjective did not come into play until the 1930’s. I would love to find out how that happened. Before it had always been used as a noun meant to describe a hard, strong rock.
I didn’t discover anymore in my Google search, but in the Oxford English Dictionary I found enough to suggest to Oxford’s Fowler that they change their date of adamant’s first usage for “something unshakable or inflexible,” or add that it didn’t become well-known, or in more common use, until the 1930’s.
Listed below, thanks to Oxford English Dictionary, are some uses that pre-date the thirties (in one instance, the eighteen thirties);
At times I feel my nerd self becoming adamant about wanting Oxford English to cross check with Oxford Fowler, but only when I wear my Adamant mask. At all other times I am happy just reporting.