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.

In Torchwood, a TARDIS coral can be seen growing on Captain Jack's Desk, although this has not yet been fully explained.