Agree trees are awesome!

Wood the rarest material? Pfft! I know you're joking, but I'm glad you raised that things that are common on Earth can actually be rare in the universe, and vice-versa. I forget that unless reminded. There must be zillions of times more gold than bronze in the universe.

I tried to think of the rarest thing in the universe, but decided it's probably whatever short-lived element was created recently that hasn't decayed yet. How many people in the world know whether there's any Mendelevium in existence today? There are plenty of gold and silver price-tracking websites. Is there one for Mendelevium? Commodities futures?

Thinking instead of the rarest naturally occurring material would just depend on how specific you wanted to get. Do Oak Apples just count as wood? How about Saint Helena Olive seeds?