Despite the commonality of it, obsidian would probably be your best bet. A nice rainbow piece if ya got some. I've found that anything over a 6" blade seems awkward, but if it is well displayed and the handle is the right proportion the knife can look high end with longer blades.
For blade designs, something with a sweeping point and an angled finger guard is very eye pleasing. However, there are so many options as long as you keep the flow of the handle and blade in mind you can come up with some really crazy designs that still look great! Good luck and keep us posted.