What is made of wood, but no saw on Earth is sharp enough to cut?

I think the answer is "sawdust".

This riddle is often seen with the following clue:

It's something you can't build with, but you'll find it anywhere things are built.

And that's what makes me think sawdust might be the right answer.

There could be other things made of wood that a saw would have trouble cutting through though.

Here are some other possibilities:

Methuselah - the tallest tree in the world. For its own protection, California's State Forest Service don't actually disclose its location to the public, so getting to it with a saw would be difficult.

Quebracho - It's a type of wood which gets its name from the Spanish word for "axe breaker". Literally, the world's toughest wood.

Noah's Ark - It withstood 40 days and 40 nights of the worst weather in the world. You think a puny saw is going to put a dent in it?