TARDIS coral

TARDISes are grown, not made (DW:The Impossible Planet). The 9th and 10th Doctor's TARDIS looks it, lots of pillars seem more like trees or vines than structural support, and an unnamed coral like substance is visible on some parts of it.

TARDIS coral started growing around the tribophysical waveform macro-kinetic extrapolator after it was integrated with the TARDIS long enough.

Captain Jack Harkness is growing a piece of TARDIS coral on his desk in Torchwood 3. Presumably he took it's 'seed' sometime before being abandoned on Satellite Five.