What about knots and sap -wood?

This grade looks natural yet consistent, with few knots. Up to 10% sap-wood can be found in wood boards, along with considerable grain and colour variation, but only small knots. Big part of the wood’s character is showing through large knots, dark streaks, filled knots and colour variation. 

 Large knots, splits, cracks and any amount of sap- wood make hardwood a fun choice, and mean that your floor will be all that more distinctive. Knots occur naturally in wood where a branch grows out of the main trunk. They can go deep into the timber’s core. In terms of flooring, there are two types of knot. A dead knot is where the core of the knot has fallen out or been removed, and then filled. With a live knot, the core remains in the board. 

The rustic and extra rustic grades contain both types of knots.