Lignum Vitae: "Living wood".
It is from South America and it is incredibly hard. I was given a small piece of the stuff which already was over 60 years old and had spent most of that at sea. Put it on the chopping-block and hit it once; it took the edge completely OFF 2 inches of my brand-new Sandvik axe.
It ages to a gray colour but, if you tke a very sharp blade and remove just a sliver, it is still yellow-green just under the surface.
It is heavier than water and you have to treat it like steel: holes are drilled first, then it can be nailed onto another plank.
I don't know about now, but, at one time, it was common to Greenheart-sheath STEEL ships which had to operate in heavy ice. The reason was that the ice could pierce and rip a long hole in the steel but the Greenheart just would slide along it and not be damaged. It is really amazing stuff.
It has been called a type of Ironwood. You can get it in small pieces at Windsor Plywood. For really big pieces, go to an East Coast shipyard.
Sheathing on the old TERRA NOVA was 2 inches, BTW. She made it farther South than any ship previously, all the way to the Antarctic coast: petty darned good for a century ago.
Hope this helps.