TARDIS coral

TARDIS coral was the organic substance of which a TARDIS was composed. The vessels were grown, not built. (DW: The Impossible Planet) The interior of the Ninth and Tenth Doctor's TARDIS had a coral-like appearance; there were several pillars that seemed more like trees or vines than structural support, and a coral like substance around portions of the structure.

However, when briefly merged with his tenth incarnation's TARDIS, the Doctor's fifth incarnation noted that the coral structures could be part of the current TARDIS' desktop theme. He also suggested that it was worse than the "leopard skin" appearance. (DW: Time Crash)

The the tertiary console room the Doctor's TARDIS had a similar organic rock-like appearance. (NA: Nightshade)

TARDIS coral began growing around the tribophysical waveform macro-kinetic extrapolator after it was integrated with the TARDIS long enough. (DW: The Runaway Bride)

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.)

According to the deleted scene a piece of TARDIS coral, which appeared to be roughly the size of a mouse, would take thousands of years to grow a TARDIS.