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.

Behind the Scenes
Part of the Bad Wolf Bay scene at the end of (DW:Journey's End) was removed in which the Doctor gave the Meta-Crisis Tenth Doctor and Rose a piece of TARDIS coral, and Donna told them how to increase the rate of growth by a factor of 59. (This scene was removed due to complicating Rose's departure, but it will be included in the DVD box set.)