Are there any in-universe or out-of-universe descriptions of the material that the dedication plaques are made from?
Assuming you're happy with Extended Universe sources...
We have confirmation in Strange New World 10 - Second Prize that the dedication plaque of the USS McAuliffe is made out of brass.
She ran her fingers across the raised letters of the brass dedication plaque adorning the wall. Jillian liked the U.S.S. McAuliffe, though the air temperature was a bit chilly for her taste.
And in Star Trek: The Captain's Table - Darkness we learn that the dedication plaque of the USS Stargazer is bronze.
Even in the twenty-fourth century, some people acted on the basis of superstition. Jovinelly’s compelled her to touch the bronze dedication plaque that hung near the turbolift on the Stargazer’s bridge.
As a shuttle specialist working under Lieutenant Chang, she didn’t get up there very often. But when she did, she ran her fingertips over the plaque before she went about her business.
To bring light into the darkness. Those were the words inscribed there, the burden with which ship and crew were charged.