Try Azobe, they sell whole railway sleepers of it, like lignum vitae its one of the few woods that doesn't float having a specific gravity higher than 1 ,
[URL unfurl="true"]https://www.uk-timber.co.uk/new-untreated-railway-sleepers/109602-new-azobe-ekki-untreated-railway-sleepers.html[/URL]
Its a very hard wood so your tooling will be tested but it resists knocks etc very well. Its also a very fine grain so the cross grain part of the ring will remain solid and stable.
[URL unfurl="true"]https://www.mytropicaltimber.org/en/c/6/products/13/azobe[/URL]