It's not Koa. I believe it is "Koai'a" which is denser than Koa. I'll have to follow-up with my brother-in-law just to make sure. He is from Hawaii and had been holding on to this piece of wood for a number of years. I was told this wood was used to make shark tooth clubs, daggers, fish hooks etc. You wouldn't want to try to build a boat out of it because it sinks like a rock!
Lee