I found that for natural looking substarte chipped coconut bark was the best it is very absorbant, good for increasing humidity without having a damp bottom, leaves not powdery residue and has no sharp edges, but it is rough enough for the snake to shed against it. Coconut oil is also a natural moisturiser and antifungal so it doesn't get mouldy.

It also holds the heat well so my snake likes to bury itself in it. It is unfortunately rather expensive as substrates go.