Actually my guess would be it was platinum plated. There were a couple "sellers" that plated the
State Quarters in gold, and in silver and sold them for big mark ups with a "presentation case", commonly known as a wooden box.
I mean it could be nickel or chromium, but more likely they are gold, or in this case, platinum plated as it was more common than nickel or chromium as far as the marketing at the time.
theres a set of 1 P and 1 D of this design from 1999 and then a P and a D of the State park quarter from 2017 sold for $9.95 called like 50
State Quarter platinum edition, and each state, 4 coins, plated in platinum, roughly $10 +shipping and handling, so easily $500 for the complete set for about $200 in materials to market it for all 50 states.
And they really make the money on the shipping and "handling", making a couple dollars for each issue off the shipping for doing nothing and get people on a subscription for the complete set.... anyways. It's more than likely platinum plated if anything.