White Bronze grave markers are actually made from zinc. The fancy white bronze name was a marketing ploy to make the zinc material sound more attractive. The white bronze markers came from the Monumental Bronze Company, Bridgeport, Connecticut and its later subsidiaries. The markers were sold nationwide. General timeframe for these markers is 1870s to 1912.
The colors can be pale grey to a very pale baby blue. The markers are hollow so there will be vertical seams at the corners. There are stories of how gardeners hid garden tools in tall white bronze monuments and how outlaws hid their loot in them.
Sometimes poor quality repairs are made to zinc markers by using steel screws, usually as replacements for missing screws. The steel screws can rust and cause discoloration. These zince markers have stood the test of time better than their neighboring contemporary stone markers eaten away by lichens and mosses.