Ipe wood, which is also sold under the name Pau Lope. It is very heavy and hard--so dense that the National Forest Products Assn and the Uniform Building Code give it a Class A-1 fire resistance rating.
I just saw a walkway that was the main/front entrance into a large commercial building that was made of ipe and it looked spectacular after a couple years of wear and weather. If it looked that good in that kind of service, it should be even better in residential use. This was installed with 3 stainless screws on about a 3" or so wide board width.