There is a fungus called Fistulina hepatica (the beefsteak fungus) that occasionally colonises the roots of an oak. It slowly causes a reaction with the tannins found with oaks. Tannins have been extracted from oak and particularly oak bark for centuries to be used in the tanning/preserving of leather; this is one of the reasons the oak is so durable and renowned for being able to withstand the vagaries of the British climate, not to mention the wooden walls of England’s ships of the line.
The reaction between the tannins and the beefsteak fungus starts with streaks of darker colour permeating the otherwise pale honey colour of the oak’s grain, it then progresses through a brindle oak stage until it becomes universally dark and it is then called brown oak. At each stage the timber merchants will wax it with words and increase their price. This is understandable because it transforms oak from beautiful to exceptionally beautiful.